Modules: add opentelemetry-proto module with protobuf definitions

[inge4pres]

Reason for this PR

Closes #19

Details

We add a new folder in the monorepo, hosting the module with the protobuf-generated code.
Same dependencies and buld steps are ported to the main build.zig.
A new build folder component is made for the protobuf module.

Documentation is updated to reflect the multiple modules structure.

[inge4pres]

For testing this PR you will need to sync the submodule first.

git submodule update --init opentelemetry-proto/proto-src

and then you can run the build steps

zig build proto-generate
zig build proto-test

[agagniere]

Why use a git submodule here ? we could add it as a lazy dependency in the build.zig.zon

[inge4pres]

Why use a git submodule here ? we could add it as a lazy dependency in the build.zig.zon

@agagniere good question 👍🏼
It's what has been used in the previous repo because we wanted to be able to pin it to tagged versions.

Can we do the same with lazy dependencies? I genuinely haven't used them.
Does a lazy dep require a build.zig/build.zig.zon in the remote repo?