Isolate OHTTP Gateway in payjoin-directory

[DanGould]

Isolate OHTTP Gateway operation to a separate abstract module from payjoin-directory/src/lib.rs so that it serves as a reference for other implementations like Cashu Dev Kit, Nostr Relays etc. It should separate support for our x-ohttp-allowed-purposes mechanism and ohttp_keys and and ohttp decapsulation from the main lib file.

After this is done, investigate if this could be made into a tiny gateway crate.

[zealsham]

i"ll work on this @DanGould

[DanGould]

Beware that reviewing this may not be top priority before payjoin-1.0 milestone closes, but I do think you have the chops to take it on after having worked on the metrics.