Stabilize Payjoin v2 with secp256k1 HPKE in OHTTP

[DanGould]

The ohttp crate does not yet support secp256k1 keys. Since payjoin is designed for bitcoin settings, using secp256k1 and especially the common libsecp256k1 crate would allow us to reduce dependency burden downstream.

Using an HPKE crate that depends on libsecp256k1 would be of use for both this issue and #125

tagging @dongcarl @moneyball