Add `SqlAlchemySnapshotRepository`

[garciparedes]

Here is an usage example:

snapshot = SqlAlchemySnapshotRepository.from_config(config)

session = await snapshot.session()
PriceTable = await snapshot.get_table(PriceEntity)
ProductTable = await snapshot.get_table(ProductEntity)

session.query(PriceTable).where(PriceTable.value > 234).join(ProductTable)