feat(meta): values symmetric encryption

[fabiobozzo]

Add encrypted metadata support to UCAN tokens

This PR adds support for encrypting sensitive metadata in UCAN tokens using AES-GCM symmetric encryption. This allows tokens to carry both public and encrypted metadata, where encrypted values can only be read by parties that possess the encryption key.

Changes

• Added AES encryption/decryption utilities in pkg/crypto/aes.go
• Extended Meta type to support encrypted values via AddEncrypted and GetEncryptedString/Bytes methods
• Added comprehensive test coverage for both string and bytes encryption
• Maintained backward compatibility with existing unencrypted metadata

Example Usage

// Create a token with both public and encrypted metadata
token, err := delegation.New(privKey, aud, cmd, pol,
   delegation.WithMeta("public", "visible to everyone"),
   delegation.WithEncryptedMeta("secret", "sensitive data", encryptionKey),
)

// Later, retrieve the encrypted value
secret, err := token.Meta().GetEncryptedString("secret", encryptionKey)

Security Notes

• Uses AES-GCM for authenticated encryption
• Each encryption operation uses a unique nonce
• Encrypted values are stored in their encrypted form in the token's metadata
• A nil encryption key results in storing the value unencrypted (for backward compatibility)