Represents a signature algorithm and other information
necessary to verify signatures with a given public key. This is
based primarily on the public key types supported by Tink's
PemKeyType, which is in turn based on KMS's supported signing
algorithms. See https://cloud.google.com/kms/docs/algorithms. In
the future, BinAuthz might support additional public key types
independently of Tink and/or KMS.
