Builds the RFC 9421 §2.5 signature base — the exact byte string that is
signed and verified. This string is THE interop debugging artifact;
MessageSignatures.signature_base/3 exposes it publicly.
Summary
Functions
@spec build( MessageSignatures.Message.t(), MessageSignatures.SignatureParams.t(), keyword() ) :: {:ok, String.t()} | {:error, MessageSignatures.Error.t()}