Cleanup `Sender::endpoint`

[spacebear21]

I looked to see if this was used anywhere in the v1 app. It's not and thus should not pub. The wrapper method is used in the v2 app, but only to identify the session in clear_send_session. This has got to be cleaned up. Perhaps v1 needs to be pub(crate) and the v2 iteration of this needs to return token instead in the next iteration of persistence if such a method is necessary at all.

Originally posted by @DanGould in https://github.com/payjoin/rust-payjoin/pull/622#discussion_r2023870126

[DanGould]

@spacebear21 and I discussed getting this in before 0.23 but decided against it because it seems to involve a change to our persistence API