Use dynamic version provider

[sleberknight]

• Use a VersionProvider instead of hard coding the version in ChangelogGeneratorMain
• In unit tests, Package#getImplementationVersion will always return null, so the code returns "[unknown]" in that situation. It allows a "test" even though it's not exactly awesome.

Closes #152

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
83.3% Coverage on New Code
0.0% Duplication on New Code

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