[3.8.x][MNG-7395] - Support system-properties in extension.xml

[laeubi]

We at Tycho have a combination of a core-extension and a build-plugin. What always is a source of confusion, that one is able to define e.g. a <tycho-version> property that could be overridden on the commandline using -D but no such thing exits for the .mvn/extensions.xml

This makes it also hard to make some regression-testing e.g. to check fast if a certain problem occurs also in the previous version as one always has to take in mind that the .mvn/extensions.xml must be edited by hand.

For that reason it would help if at least basic interpolation using the "maven-properties" would be possible.

This PR adds support for having basic interpolation in .mvn/extensions.xml to either:

1. Specify a (default) version in maven.config
2. Specify a version on the commandline with mvn -Dsome.version=1.2.3

FYI @mickaelistria @akurtakov

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[michael-o]

@laeubi Instead of or additionally jvm.config you can also use maven.config. This will make it a user property.

[laeubi]

@michael-o thanks for the hint with maven.config! I'm currently try to derive a Integration test and probably will use maven.config there as well.

[michael-o]

Good, jvm.config should really be for JVM-related options only.

[laeubi]

I have updated the PR description accordingly.