feat: add "use_interface_return_types" option

[Jesse0Michael]

• Add the option "use_interface_return_types" to direct sqlc generation to use the querier interface for the return type of the WithTx method on the Queries struct.

• Include WithTx in the Querier interface (when "emit_methods_with_db_argument" is false)

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Looks to resolve https://github.com/sqlc-dev/sqlc/issues/383

Where the lack of WithTx in the interface causes makes it hard to use the Querier interface and the *Queries return type makes it hard to mock the sqlc Querier for testing

Please let me know if I should have approached this differently or if there's something else I need to do! ✌️

[Jesse0Michael]

It would be nice if we could just update the WithTx signature to use "Querier" instead of "*Queries" if emit_interface is enabled.. but, as @kyleconroy mentions in the issue https://github.com/sqlc-dev/sqlc/issues/383#issuecomment-596053061, that would be a breaking change.

My hope is that we could put the change behind an explicit option.

I imagine use_interface_return_types isn't a clear enough name for an option that would fix the function signature of WithTx, but instead of guessing what would be a clearer name, I'd rather be told would be an acceptable option name