OpenSSL3: Implement EdDSA and XDH; refactor key agreement; wire provider

[ariawisp]

What

• Add OpenSSL3 implementations for EdDSA (Ed25519/Ed448) and XDH (X25519/X448).
• Refactor key agreement ops and add nullable-hash handling for digest signatures.
• Wire new algorithms in Openssl3CryptographyProvider.
• No CI/build/test changes in this PR.
• One-shot digest signatures now use AccumulatingSignFunction/AccumulatingVerifyFunction.

Why

• Provide native provider support for EdDSA/XDH behind the new core APIs; unblocks cross‑provider validation.

Notes

• Depends on Core APIs (EdDSA/XDH) — blocked by PR Core: Add EdDSA and XDH algorithm interfaces #109.
• Optional DER/PEM support can follow once ASN.1 changes merge (PR ASN.1: RFC 8410 OIDs, ABSENT params, Unknown key preservation, SEQUENCE OF support #108); this PR focuses on provider implementation and wiring.