Add an existing wallet to Gateway using its private key. Gateway handles encryption and storage internally.
Args: wallet_credential: Wallet credentials (chain, private_key, and optional set_default)
Returns: Wallet information from Gateway including address
Raises: HTTPException: 503 if Gateway unavailable, 400 on validation error
Basic authentication header of the form Basic <encoded-value>, where <encoded-value> is the base64-encoded string username:password.
Successful Response