feat(diag) expose unparseable config errors

[rainest]

What this PR does / why we need it:

Add a diagnostic endpoint for raw config errors to simplify problem identification when Kong does not return flattened errors.

Attempt to log the message field if we cannot unmarshal a config error into the proper type.

Which issue this PR fixes:

Fix #5762

Special notes for your reviewer:

Logs from practical testing:

2024-03-28T19:45:41Z	error	could not fully parse config error	{"message": "declarative config is invalid: {targets={[2]=\"uniqueness violation: 'targets' entity with primary key set to '172fcf76-2116-5155-ae84-e534423f5354' already declared\"}}"}
2024-03-28T19:45:41Z	error	Failed parsing resource errors	{"url": "https://10.244.0.6:8444", "update_strategy": "InMemory", "error": "could not unmarshal config error: json: cannot unmarshal object into Go struct field ConfigError.flattened_errors of type []sendconfig.FlatEntityError"}
2024-03-28T19:45:41Z	error	dataplane-synchronizer	Could not update kong admin	{"error": "performing update for https://10.244.0.7:8444 failed: failed posting new config to /config: got status code 400\nperforming update for https://10.244.0.6:8444 failed: failed posting new config to /config: got status code 400"}

Diag output looks like endpoints.txt using resources in test.yaml.txt

---

I wanted to log the basic message field as well, to provide a more accessible (no need to enable config dumps) record of mystery errors. The body observed in the original report does have a message field:

{                      
  "message": "declarative config is invalid: {targets={[2]=\"uniqueness violation: targets entity with primary key set to 0500bbf6-db53-52da-ad42-d13937d1e29c already declared\"}}",
  "flattened_errors": {},
  "code": 14,
  "fields": {
    "targets": [
      null,
      "uniqueness violation: targets entity with primary key set to 0500bbf6-db53-52da-ad42-d13937d1e29c already declared"
    ]
  },
  "name": "invalid declarative configuration"
}

We unmarshal into a struct that has a message field and omits the rest if missing, but it's still failing for some reason. The lazier map[string]interface{}{} unmarshal hopefully handles this if we've at least gotten a Kong JSON config error and not a generic 500 or whatever.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: Patch coverage is 47.91667% with 25 lines in your changes are missing coverage. Please review.

Project coverage is 74.0%. Comparing base (4954d25) to head (b312ebb).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...al/dataplane/sendconfig/inmemory_error_handling.go	0.0%	8 Missing and 1 partial ⚠️
internal/diagnostics/server.go	18.1%	9 Missing ⚠️
internal/dataplane/sendconfig/dbmode.go	20.0%	4 Missing ⚠️
internal/dataplane/sendconfig/sendconfig.go	80.0%	2 Missing ⚠️
internal/dataplane/sendconfig/inmemory.go	66.6%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5773     +/-   ##
=======================================
+ Coverage   71.4%   74.0%   +2.6%     
=======================================
  Files        176     176             
  Lines      18185   18207     +22     
=======================================
+ Hits       12986   13480    +494     
+ Misses      4248    3733    -515     
- Partials     951     994     +43     

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[rainest]

1b91b2c and the changelog were supposed to be there originally but I forgot to push the commit

[rainest]

Accidentally included some local temp change I'd made to make a test run forever so I could check some things 🤦