fix(sharding): apply default mapping for any table

[meskill]

There could be some discrepancies for sharding depending on configuration that is not very obvious.

Here is the before and after map.

[[sharded_tables]] name	[[sharded_mappings]] table	Before	After
absent	absent	✅ mapping applied	✅ mapping applied
"users"	"users"	✅ mapping applied	✅ mapping applied
"users"	absent	❌ hash routing - no mapping	✅ mapping applied
absent	"users"	❌ hash routing - no mapping	❌ not fixed yet

I wanted to verify if that what do we actually want, or do we want to apply mapping directly to the sharded_table configuration and require specific table/mapping to be defined in order to it to be merged together.

I'd think about fix both cases in this pr and apply catch-all settings for any other specific configuration or add the validation for configuration to prevent misuse.

[codecov]

Codecov Report

❌ Patch coverage is 85.12397% with 36 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/router/sharding/test/mod.rs	84.25%	34 Missing ⚠️
pgdog/src/backend/databases.rs	92.30%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[levkk]

Kinda complicated, but I get it. I think we may want a more tight config for sharding here. name is really optional if the column is the same in all tables, for example. We may even want to provide some kind of config builder based on the schema and users' input.