This PR introduce a direct type conversion from PG to CH. This has the benefit of not exposing qvalue, which ideally is hidden as implementation details inside peerdb.

• This logic is added to a shared package (flow/shared/) for easy import.
• Added a unit test to ensure that the direct type mapping (PG type -> CH type) returns the same result as indirect type mapping (PG type -> qkind -> CH type). Currently the postgres oids are hard-coded, as I did not find a pgx lib call that allows for exhaustively iterating through the pg types, but we can turn this into an integration test and use a real postgres backend to get exhaustive pg types.
• Currently the supported destination types is hard-coded in flow/shared, this means it's possible that ToSupportedDestinationTypesForPostgres method in flow/shared/clickhouse/type_conversion.go and supportedDestinationTypes in flow/connectors/clickhouse/type_conversion.go are out-of-sync, resulting destination type conversion getting ignored if that happens. We could add a test to ensure this is in sync as well (open to other ideas).

TODO:

• test this e2e in CP.
• sort through the edge cases with type modifier and custom types
• replace old fetchAllTypeConversions with a new fetchTypeConversionsForColumns endpoint using direct type conversion
• update peerdb ui to use new logic
• remove qkind from column type once it's no longer needed