Payjoin directory should persist OHTTP keys

[spacebear21]

Currently it generates new keys on startup, which means anytime the directory server is restarted it produces new OHTTP keys, and any existing payjoin sessions using that directory fail due to " Bad request: Key configuration rejected".

The directory should check if a "ohttp_keys" file exists, and use the existing key configuration if it exists, or generate a new one and save to file otherwise.

Relates to #471

[spacebear21]

Perhaps the directory should have the ability to support overlapping key configurations, e.g. "ohttp_keys.old" can be accepted for some grace period while clients switch to "ohttp_keys".

[DanGould]

Indeed, the RFC even makes this explicit "More than one configuration might need to be active to allow for key rotation and server maintenance." @ privacy considerations