Implement P-256 Support in OPAQUE Protocol

[kevinamccorvon]

OPAQUE protocol needs P-256 curve support. There's already a TODO marker in hpcrypt-kex/opaque.rs noting this gap.

P-256 is everywhere in industry standards, so adding it will make the library play nicer with existing systems and other OPAQUE implementations.

TODO:

• Implement P-256 support in OPAQUE
• Test compatibility with current implementation
• Add P-256 test vectors
• Write usage examples in docs