Support using a pre-existing sqlx pool for postgres (and others as appropriate)

[derekleverenz]

SQLx provides database pool implementations of postgres and other dbs and is commonly used in the rust ecosystem. In the case of using datafusion-table-providers to expose postgres in datafusion, but also doing other postgres operations with sqlx, it would be ideal to have datafusion-table-providers use the same pool as the rest of the application.

It looks like it would be possible to either replace the underlying bb8 pool with a sqlx one or abstract it so that either could be used.

I'm happy to take a stab at implementing this myself but want to get feedback before I go and do the work

[phillipleblanc]

Yeah, the PostgresConnectionPool struct in src/sql/db_connection_pool/postgrespool.rs encapsulates the BB8 pool and returns instances of PostgresConnection in src/sql/db_connection_pool/dbconnection/postgresconn.rs that wraps the actual bb8::PooledConnection.

I don't see much point having runtime code to select between the two, so it might be better to add a new feature sqlx that when enabled switches the PostgresConnectionPool/PostgresConnection to use the sqlx-based pool.

Happy to review any draft/in-progress code once you have an approach you'd like feedback on.

[derekleverenz]

Looks like this will be a bigger task than I thought initially, using sqlx will also involve replacing the tokio_postgres client and redoing the rows_to_arrow implementation for postgres. Still doable I think just more involved