feat: support for external signers in NIOSSHPrivateKey

[camunro]

Support for External Signers in NIOSSHPrivateKey

Motivation

This PR enables hardware-backed authentication and remote identity protocols by allowing NIOSSHPrivateKey to delegate signing operations to external providers (e.g., Secure Enclave, TPM, or ssh-agent) without requiring raw private key material to be loaded into memory.

This is a foundational architectural change that enables several high-level features for the Swift ecosystem:

• SSH Agent Protocol: Provides the necessary hooks for a client to list keys from ssh-agent and delegate the sign request back to the agent. (Add SSH Agent Support #189)
• Hardware Security (Secure Enclave / TPM): Allows keys stored in the Apple Secure Enclave or a TPM to sign SSH challenges via Biometrics (TouchID/FaceID).
• PKCS#11 / Smart Cards: Enables integration with physical security tokens and other non-exportable identities.

Modifications

• New Protocol: Defined NIOSSHExternalSigner, which defines a standardized interface for delegated signing operations.
• NIOSSHPrivateKey Evolution: Added an .external(NIOSSHExternalSigner) case to NIOSSHPrivateKey for all supported algorithms:
  • Ed25519
  • ECDSA (NIST P-256, P-384, and P-521)
• Error Handling: Added NIOSSHError.externalSignerFailure to provide clear feedback and diagnostic info when an external delegate fails.
• Documentation: Updated Docs.docc to provide guidance on implementing external signers for hardware-backed security.
• Testing: Added NIOSSHExternalSignerTests.swift covering:
  • Mock external signers for all supported curves.
  • Verification that signatures produced by delegates are correctly handled by the SSH state machine.
  • Validation of error propagation when the external signer throws.

Result

NIOSSH now supports public-key authentication where the signing operation is handled by an external delegate. This provides the critical architectural hook required for modern identity-aware SSH implementations (like #189) on both Apple and Linux platforms.