CryptoKit: Implement EdDSA and XDH; add Swift sources; wire provider

[ariawisp]

What

• Add CryptoKit implementations for EdDSA (Ed25519) and XDH (X25519).
• Add Swift sources (SwiftEd.swift) used by the provider.
• Wire algorithms in CryptoKitCryptographyProvider.
• No Swift interop/Gradle task changes in this PR.
• Decoding for DER/PEM unwraps to RAW (SPKI/PKCS#8) and then calls decodeRawWithRawRepresentation (public/private) per maintainer guidance.
• Internal naming: rename inner types to EdPublicKey/EdPrivateKey and XPublicKey/XPrivateKey.

Why

• Provide Apple platform support for Ed25519/X25519 behind the new core APIs.

Notes

• Depends on Core APIs (EdDSA/XDH) — blocked by PR Core: Add EdDSA and XDH algorithm interfaces #109.
• Uses ASN.1 OID constants — blocked by PR ASN.1: RFC 8410 OIDs, ABSENT params, Unknown key preservation, SEQUENCE OF support #108 for DER/PEM-related paths.
• Capability note: CryptoKit typically supports Ed25519/X25519 only (not Ed448/X448).
• No CI/build/test changes here; any SwiftInterop/Xcode adjustments, if needed, will be proposed separately.

[whyoleg]

@ariawisp, thanks for the PR! I've finalized the changes and merged them into master (with attribution).