166 HQDM Extensions Mechanism

[twalmsley]

See HQDM Model Extension Example for how to write an extension module.

(We could perhaps copy the example into the MC repo or create a better example.)

[GCHQDeveloper42]

This functionality could do with unit testing, but seen as we don't have a test suite for HqdmObjectFactory yet, that may be asking too much of this PR.

[twalmsley]

• I added a module containing the example extension (it's a separate module so that it gets built to a JAR for use by the examples module).
• I wasn't able to get a JUnit test working in the examples module so I added it to src/main/java instead as a main program which runs successfully. There are instructions about how to run it in the class javadoc.

Perhaps at some point we can create a proper unit test if that's possible, but right now I think this might have to suffice.