Core: Add EdDSA and XDH algorithm interfaces

[ariawisp]

What

• Introduce EdDSA (Ed25519/Ed448) and XDH (X25519/X448) APIs under algorithms/.
• Update core API descriptors to include the new types.
• No provider or test changes in this PR.

Why

• Establish a stable API surface for providers to implement and for tests to target, independent of provider wiring.
• Follows ASN.1 prep so providers can import/export keys consistently.

Follow‑ups

• Provider implementations (JDK, OpenSSL3, WebCrypto, CryptoKit) will wire these APIs.
• Tests add default/compatibility suites for these algorithms.