Support explain diagnostic command

[seeforschauer]

Description

This PR closes #167.

PR checklist

• convert returned JSON document into types.Document of the same form;
• implement explain command:
  • verbosity parameter should be ingored;
  • explainVersion, command, serverInfo fields should be returned as in MongoDB;
  • queryPlanner should contain converted EXPLAIN (FORMAT JSON) results;
  • executionStats and serverParameters should be absent;
• change fetch method to run EXPLAIN (VERBOSE true, FORMAT JSON) <query> instead of the query itself if parameter is given;
• write integration test;
• refactor fetch method (split it into two?) if it seems to be needed on review.

Example output

ferretdb=#
ferretdb=# EXPLAIN (FORMAT JSON) SELECT _jsonb FROM "testcommandsdiagnosticexplain"."testcommandsdiagnosticexplain_1dd9d237"
ferretdb-# ;
                            QUERY PLAN
------------------------------------------------------------------
 [                                                               +
   {                                                             +
     "Plan": {                                                   +
       "Node Type": "Seq Scan",                                  +
       "Parallel Aware": false,                                  +
       "Async Capable": false,                                   +
       "Relation Name": "testcommandsdiagnosticexplain_1dd9d237",+
       "Alias": "testcommandsdiagnosticexplain_1dd9d237",        +
       "Startup Cost": 0.00,                                     +
       "Total Cost": 23.60,                                      +
       "Plan Rows": 1360,                                        +
       "Plan Width": 32                                          +
     }                                                           +
   }                                                             +
 ]
(1 row)

Or

--------------------------------------[ RECORD 1 ]--------
QUERY PLAN 
| [                                        +
|   {                                      +
|     "Plan": {                            +
|       "Node Type": "Seq Scan",           +
|       "Parallel Aware": false,           +
|       "Async Capable": false,            +
|       "Relation Name": "values_34474c3b",+
|       "Schema": "test",                  +
|       "Alias": "values_34474c3b",        +
|       "Startup Cost": 0.00,              +
|       "Total Cost": 23.60,               +
|       "Plan Rows": 1360,                 +
|       "Plan Width": 32,                  +
|       "Output": ["_jsonb"]               +
|     }                                    +
|   }                                      +
| ]

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• (for maintainers only) I set Reviewers, Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #909 (ba43932) into main (8bcfbd9) will decrease coverage by 0.71%.
The diff coverage is 20.60%.

@@            Coverage Diff             @@
##             main     #909      +/-   ##
==========================================
- Coverage   62.66%   61.94%   -0.72%     
==========================================
  Files         225      228       +3     
  Lines       10463    10591     +128     
==========================================
+ Hits         6557     6561       +4     
- Misses       3135     3254     +119     
- Partials      771      776       +5     

Impacted Files	Coverage Δ
internal/handlers/common/error.go	82.65% <ø> (ø)
internal/handlers/common/errorcode_string.go	80.00% <ø> (ø)
internal/handlers/common/msg_listcommands.go	85.71% <ø> (ø)
internal/handlers/dummy/msg_explain.go	0.00% <0.00%> (ø)
internal/handlers/pg/msg_explain.go	0.00% <0.00%> (ø)
internal/handlers/tigris/msg_explain.go	0.00% <0.00%> (ø)
internal/handlers/pg/pgdb/query_documents.go	36.55% <25.55%> (-24.37%)	⬇️
internal/types/document.go	85.63% <100.00%> (+0.51%)	⬆️
internal/clientconn/conninfo/conn_info.go	66.66% <0.00%> (-16.67%)	⬇️
internal/handlers/pg/msg_dbstats.go	65.11% <0.00%> (-6.98%)	⬇️
... and 3 more

Flag	Coverage Δ
integration	56.64% <20.60%> (-0.70%)	⬇️
mongodb	18.65% <6.74%> (-0.21%)	⬇️
pg	61.58% <20.85%> (-0.87%)	⬇️
tigris	18.17% <6.66%> (-0.20%)	⬇️
unit	24.77% <16.36%> (-0.26%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[seeforschauer]

It's not ready yet but let's discuss some:

• I've added explain parameter to iterateFetch. iterateFetch(ctx context.Context, fetched chan FetchedDocs, rows pgx.Rows, explain bool) error. Do you have any thoughts on how can it be more beautiful?
• the Convert func in internal/util/convert/convert.go. Should it be in that package or in another place?
• the test doesn't check any values. I'm about to add some, for example, from the example structures (see header)

[ferretdb-bot]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ AlekSi
❌ Ekaterina Blech

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