refactor: consolidate multi-step planning, assembly and execution into QueryPlanner type

[jkaczman]

@sgrif and I have been working on #1096 (subqueries), and as the first step to getting there, discussed how we can create an architecture for multi-step that...

• (1) reduces duplication,
• (2) concentrates all relevant code into one area,
• (3) clearly articulates query dependencies (as in, we cannot construct a query without receiving the responses of a previous query). An example of being we cannot construct the INSERT for Sharding Key Updates without the DELETE ... RETURNING * DataRow returning. Another example being, of course, subqueries.
• (4) doesn't do more than necessary; as in, we only need the Parse/Bind handled dynamic Steps, we don't need to pre-make requests for each before execution-time,
• (5) removing unnecessary code, e.g. the InPlace variant, so that we can just receive the QueryPlanner and handle it, and also, making OffsetPlan independent of this new struct, as it's not relevant (and can just be passed straight through)
• (6) a nice abstraction for (1) knowing when to forward messages to the Client, and (2) determining how to aggregate them (especially in dynamic situations)
• (7) running checks during planning instead of having a check in the parser and deferring more checks until later, which we can do because we have the QueryEngineContext to work with now.
• ... and more

No tests were removed. Added a couple of tests for a few things I wanted to make sure worked.

A lot of the diff is from moving pre-existing functions/tests around.

• pgdog/src/frontend/router/parser/rewrite/statement/update.rs => pgdog/src/frontend/client/query_engine/multi_step/test/sharding_key_update.rs
• pgdog/src/frontend/router/parser/rewrite/statement/insert.rs => pgdog/src/frontend/client/query_engine/multi_step/test/split_insert.rs

TODO: Cache insert splits / subqueries / etc (relevant benchmark: #1484)

[codecov]

Codecov Report

❌ Patch coverage is 97.91901% with 37 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ntend/client/query_engine/multi_step/ops/update.rs	96.41%	12 Missing ⚠️
...ntend/client/query_engine/multi_step/ops/insert.rs	93.00%	10 Missing ⚠️
.../frontend/client/query_engine/multi_step/shared.rs	97.87%	5 Missing ⚠️
...uery_engine/multi_step/test/sharding_key_update.rs	99.27%	3 Missing ⚠️
...lient/query_engine/multi_step/test/split_insert.rs	98.55%	3 Missing ⚠️
pgdog/src/frontend/client/query_engine/context.rs	83.33%	1 Missing ⚠️
...c/frontend/client/query_engine/multi_step/error.rs	85.71%	1 Missing ⚠️
pgdog/src/frontend/client/query_engine/query.rs	85.71%	1 Missing ⚠️
...d/client/query_engine/test/rewrite_insert_split.rs	94.73%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[levkk]

We used to cache the rewrite previously. Now, we're performing it on each request, prepared statement or not. I suspect that would add some performance overhead to this step, worth benchmarking.