Consider serializing compressed keys in ?pj= parameters

[DanGould]

Secp256k1 keys for HPKE (and OHTTP that depends on it) are 65 bytes according to the IANA DHKEM(secp256k1, HKDF-SHA256) spec. Since these need to be serialized in a sharable format often exchanged via QR, a better user experience would compress them

Compressed Public Key: A secp256k1 compressed public key consists of 33 bytes. The first byte indicates the parity (even or odd) of the y-coordinate (0x02 for even and 0x03 for odd), and the remaining 32 bytes represent the x-coordinate of the elliptic curve point. The y-coordinate is omitted in the compressed form because it can be derived from the x-coordinate and the parity.

We need to first determine whether this is a hard requirement since it adds complexity and then how we'd de/serialize them into an Ohttp Key config with default parameters that shouldn't change.

Depends on #126