Generate GTFS Realtime protobuf files in separate Maven module

[t2gran]

Summary

Generating the GTFS Realtime Protobuf files in a separate module has several advatags:

• The config is clear (what config apply this module) - pom.xml
• Speed up builds - this does not need to build every time

Issue

Part of #6155

Unit tests

No

Documentation

No

Changelog

No

Bumping the serialization version id

No

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.92%. Comparing base (0ae34a8) to head (d562a52).

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #6184   +/-   ##
==========================================
  Coverage      69.92%   69.92%           
+ Complexity     17736    17735    -1     
==========================================
  Files           1998     1998           
  Lines          75419    75419           
  Branches        7718     7718           
==========================================
  Hits           52737    52737           
+ Misses         20004    20003    -1     
- Partials        2678     2679    +1     

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[abyrd]

Based on comments in the diff, this PR seems to be about using gtfs-realtime-protobuf to "compile the gtfs-realtime.proto file to Java code". Can I update the title and description of the PR to reflect this? It currently says it's generating protobuf files, which caused me some confusion when first approaching it.

[abyrd]

Can this section in the top-level POM be moved into the submodule?

 <build>
        <extensions>
            <!-- makes build variables like os.detected.classifier available.
                 use to select the correct protoc instance for the OS.
                 see protobuf-maven-plugin below -->
            <extension>
                <groupId>kr.motd.maven</groupId>
                <artifactId>os-maven-plugin</artifactId>
                <version>1.7.1</version>
            </extension>
        </extensions>
...
</build>