Support for DynamicMessage to handle unknown types at runtime

Like this:
https://pkg.go.dev/google.golang.org/protobuf/types/dynamicpb?tab=doc

Start by reading the linked google.golang.org/protobuf/types/dynamicpb documentation and inspect the repository's existing message APIs for the appropriate entry point. Define the runtime behavior needed for unknown types and identify tests that can demonstrate it; done means the behavior is supported with coverage.

Written by the indexing model from the issue text.