Add support for top-level statement descriptor for all sources

[remi-stripe]

This removes the type-specific statement descriptor as it is now supported as a top level parameter and moves every type to StatementDescriptor.

Fixes #1088

r? @ob-stripe
cc @stripe/api-libraries

[AFallad]

Hello, I did some testing and I don't think that the type-specific statement_descriptor should be deleted, since the top level and type-specific can be different, check this request

Regards!

[remi-stripe]

@AFallad even if both can be passed to be backwards compatible, the ideal[statement_descriptor] would be ignored and we'd default to statement_descriptor instead. That's why I think it's cleaner to remove all the other ones and only officially support the top-level one.

[AFallad]

More than backward compatibility I was worried that with that change we will be unable to set the type specific ideal[statement_descriptor] which is set separately from the top level statement_descriptor by the Stripe API.
I don't know what use case would require the dev to set a different descriptor for both, but by removing the specific ones, that use case would not be supported by the stripe-net library but would be supported by the Stripe API
I appreciate a lot the swiftness. I'll keep an eye out for the release that fixes this. Thanks!

[remi-stripe]

@AFallad I hear you but ultimately if you were to pass both, one would be ignored silently. The top-level is the right one to set and you should ignore that the type-specific ones even exist. It's for an old compatibility reason and statement_descriptor is always the preferred/correct one.

[ob-stripe]

lgtm!

[stan-stripe]

LGTM