Cleanup & consistency improvements for `transparent-inputs` methods in `zcash_client_backend::data_api`

[nuttycom]

I think for consistency it would be fine to change this to t.mined_height < :target_height (and use target_height consistently) but I believe that there's no semantic difference here. Since this is a hotfix, I don't want to make any API changes here but I think that for an upcoming semver-breaking release it would be good to deprecate get_unspent_transparent_output and add a get_spendable_transparent_output method that follows get_spendable_transparent_outputs more closely. In addition, I would like to move all of the transparent-inputs-flagged methods out into a separate trait; it's going to be time to do a broader data_api cleanup some time soon, so I think any such changes can be deferred until then.

Originally posted by @nuttycom in #1970