Refactor ohttp keys

[DanGould]

Refactor OhttpKeys so that #300 can just focus on semantics.

• First Commit removes a clippy warning
• 2nd reduces unnecessary base64 verbosity. base64 dependency was being used where psbt.to_string was fine, and the custom configs were being built where confs could do
• 3rd removes a vestigial PartialEq implementation
• 4th disambiguates Display and Serialize roles. OHTTP KeyConfigs encoding for serialization is well specified by the rfc. We're using string representations in URLs and configs just for Payjoin V2 which is a separate thing

[DanGould]

I did a self review that re-ordered commits to put clippy first and change Error variants to exclude the word "Error" because it's redundant.