NewSigner creates a signer with the recommended signing method and a signing key bundled with a certificate chain.

NewSignerFromFiles creates a signer from key, certificate files TODO: Add tests for this method.

NewSignerPlugin creates a notation.Signer that signs artifacts and generates signatures by delegating the one or more operations to the named plugin, as defined in https://github.com/notaryproject/notaryproject/blob/main/specs/plugin-extensibility.md#signing-interfaces.

ValidateEnvelopeMediaType validetes envelope media type is supported by notation-core-go.

ValidatePayloadContentType validates signature payload's content type.