support custom backends, row loaders etc. via custom TableLoader in `sql_database`

[rudolfix]

Background
We provide several ways to customize sql_database via callback that let you ie. to rewrite the queries. Here we are allowing for a deep customization by subclassing TableLoader that is responsible for execution of the query over already reflected SqlAlchemy table. Overriding this class will allow:

• implementing additional backends (ie. adbc fast arrow readers or ibis backend)
• implementing custom row readers ie. with pagination (Add pagination to sql_table #2432)
• implementing own retry mechanisms
• very deep customization of generated queries

Implementation

1. • design an internal interface that users are supposed to override in subclasses
2. • test the interface so we do not break users' code by chaning it
3. • allow to: easy add new backends and override row loading
4. • provide two examples in tests and docs: paginated loading + adbc backend for postgres / snowflake (we may merge it into OSS if works well)