feat: add multi key type support for enclave signatures

[prateekreddy]

Add support for multiple cryptographic key types (Ed25519, Secp256k1) in enclave signature verification to enable attestations with various public key types.

Changes:

• Add KeyType enum with Ed25519 and Secp256k1 variants
• Add key_type field to Enclave struct to track signature scheme
• Add register_enclave_with_key_type() for explicit key type selection
• Maintain backward compatibility: register_enclave() defaults to Ed25519
• Add convenience functions: register_enclave_ed25519(), register_enclave_secp256k1()
• Update verify_signature() to dispatch based on key type using match
• Add public key length validation (32 bytes for Ed25519, 33/65 for Secp256k1)
• Update register_enclave.sh to support optional key_type parameter

Note: Enclave struct now includes key_type field. Existing enclaves will need to be re registered if upgraded.