repository layout seems to have strong Java Maven bias

The proto3 files are currently all kept under `src/main/proto` which seems to be due to the fact that this is the standard Java Maven project layout and Maven is used in the CI job to verify that the protos are valid (aka can be used to create and compile Java code from them).

As suggested [here](https://protobuf.dev/programming-guides/proto3/#location) I propose to move the protos to the top level of the repository because this repo's whole purpose is to provide a home for the uService definitions themselves (not any accompanying Java code).

Start by listing the proto3 files under `src/main/proto` and inspect the Maven-based CI validation configuration. Move the definitions to the repository top level, update any validation paths that depend on the old layout, and confirm the Maven check still verifies that Java code can be generated and compiled from the protos.

Written by the indexing model from the issue text.