Enable protobuf 3 support

Raising this as we were unable to leverage the use of Firebase functions because this library is required for use and has a hard dependency on the newer python protobuf 4 version.

To solve we needed to run Cloud Run directly so that we could avoid the cloud events library which is a dependency from the firebase functions SDK.

While I believe protobuf 4 support is crucial, I think a core library like this needs to do all it can to support as wide range of protobuf as it can, as there are older packages that some times require this. Especially when it's core use is within the cloud functions SDK which does in fact support protobuf 3

The issue names no files, tests, or entry points. Start by locating the project's dependency declarations and existing compatibility tests, then determine where the protobuf version requirement is enforced. Done means the library supports protobuf 3 while retaining protobuf 4 support, with tests covering both versions.

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