postgres_fdw fallback

[levkk]

Use postgres_fdw as fallback for cross-shard queries that we don't support yet.

[codecov]

Codecov Report

❌ Patch coverage is 82.81346% with 281 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/backend/fdw/postgres.rs	82.03%	67 Missing ⚠️
...og/src/backend/schema/postgres_fdw/custom_types.rs	76.78%	39 Missing ⚠️
pgdog/src/backend/fdw/lb.rs	0.00%	32 Missing ⚠️
pgdog/src/backend/fdw/launcher.rs	85.85%	28 Missing ⚠️
pgdog/src/backend/schema/postgres_fdw/schema.rs	85.71%	26 Missing ⚠️
pgdog/src/backend/pool/cluster.rs	58.13%	18 Missing ⚠️
...gdog/src/backend/schema/postgres_fdw/extensions.rs	73.43%	17 Missing ⚠️
pgdog/src/backend/schema/postgres_fdw/statement.rs	96.15%	16 Missing ⚠️
pgdog-config/src/sharding.rs	43.47%	13 Missing ⚠️
pgdog/src/frontend/router/parser/route.rs	18.75%	13 Missing ⚠️
... and 6 more

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