Add support for HPKE (ChaCha20-Poly1305 + DHKEM(X25519, HKDF-SHA256))

[brycx]

With #339 we added support for RFC 9180 DHKEM(X25519, HKDF-SHA256). However, for this to be properly usable and to be utilized in the higher-level API some time, we need to add full support for HPKE using ChaCha20-Poly1305 alongside this KEM. A single-shot API of HPKE can then be added to the higher-level API.