feat: Postgres sidecar with postgres_fdw extension for some cross-shard queries

[levkk]

This is a big one.

For cross-shard SELECT queries that we don't support yet (some aggregate functions, CTEs / subqueries without the sharding key, OFFSET x, window functions), allow to fallback to using the postgres_fdw Postgres extension with foreign partitioned tables.

We run an empty Postgres DB as a "sidecar" (subprocess of PgDog, really), and on boot, create all the foreign tables, partitioned by hash/list/range as configured in pgdog.toml. Partitioned tables are supported too, we use two-tier partitioning there, child tables use fdw.

We import custom types, schemas and extensions prior to this, so it should just work.

When a query comes in that requests to use the fdw backend manually, pgdog will send the query to the fdw connection pool which will forward it to the shards via fdw, assembling the rows just like it would normally.

This is a superpower. If this works as intended, we will achieve high 90%s Postgres compatibility in a sharded world.

[codecov]

Codecov Report

❌ Patch coverage is 71.69118% with 847 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...g/src/frontend/router/parser/query/fdw_fallback.rs	0.00%	234 Missing ⚠️
...tend/router/parser/query/test/test_fdw_fallback.rs	8.64%	148 Missing ⚠️
pgdog/src/backend/fdw/postgres.rs	79.80%	81 Missing ⚠️
pgdog/src/backend/schema/postgres_fdw/statement.rs	92.16%	52 Missing ⚠️
pgdog/src/backend/fdw/launcher.rs	78.99%	50 Missing ⚠️
pgdog/src/backend/schema/postgres_fdw/schema.rs	77.27%	45 Missing ⚠️
...og/src/backend/schema/postgres_fdw/custom_types.rs	76.78%	39 Missing ⚠️
pgdog/src/backend/fdw/lb.rs	0.00%	35 Missing ⚠️
pgdog/src/backend/databases.rs	40.47%	25 Missing ⚠️
pgdog/src/backend/pool/cluster.rs	57.14%	24 Missing ⚠️
... and 16 more

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