no external references in Proto

It would be great if the Proto files in this repository did not reference or import external proto files.

At the moment, some (missing) files are part of the dependencies that are downloaded during the Maven Build. However, this has some shortcomings when working with another programming language like Rust in my case.

An example setup would be to reference this uservices repository as sub-module in Git and then integrate the generation of the Rust stubs as part of the build process (in case of Rust cargo).
At least, in that scenario the following two files are missing:
```
google/rpc/status.proto: File not found.
uprotocol/uoptions.proto: File not found.
```

I understand that it is not the best idea to replicate the same files in this repository. One potential resolution could be to create a script which allows the developer to download the required missing proto-files from the respective sources and could be integrated in any kind of build automation.

What do you think?

Start by inspecting the Proto files, the Maven build, and the missing dependency paths google/rpc/status.proto and uprotocol/uoptions.proto. Determine how external references are currently downloaded and how Rust stub generation would consume them. Done means the dependency handling works without unavailable external references and is usable in another build workflow.

Written by the indexing model from the issue text.