Add support for DHKEM(P-256, HKDF-SHA256)

[m-kus]

Summary

• Add P-256 KEM (ID 0x0010) alongside existing X25519 (ID 0x0020) for OHTTP key encapsulation
• Only rust-hpke backend changed, NSS backend is not updated (does not support P-256 yet)

Most KMS providers support P-256 but not X25519. Adding P-256 KEM support enables OHTTP gateways to store and manage their keys in hardware-backed KMS, ensuring more secure key management in production deployments.

Changes

• hpke.rs: Add P256Sha256 variant to Kem enum (n_enc=65, n_pk=65, per RFC 9180 Table 2)
• Cargo.toml: Enable p256 feature on the hpke crate dependency
• rh/hpke.rs: Add P-256 key types, sender/receiver context enums, dispatch entries, and key generation