Add support for getting db session form context

[rodionstolyarov]

sqlc already have an option to emit the methods with db argument emit_methods_with_db_argument, but sometimes it is more convenient to keep the current db session in the context. It would be nice if sqlc could extract db from the context itself.

To do this, we need to pass a function that knows how to get db form the context to the New method of Querier and then use this function in all the methods generated.

This PR adds support for a new option get_db_from_context that implements this feature.