cmake: create integration for code generation

[benedekkupper]

These changes make the protobuf code generation and build steps all managed by cmake, no additional steps are necessary. Dependent projects can

1. add this as a subdirectory, or use FetchContent
2. link to EmbeddedProto static library
3. generate and include the protobuf headers (e.g. as done in the ./CMakeLists.txt file) with generate_embedded_proto
4. define custom target, which depends on these headers, and add the dependency to your project