Add basic support for GitRepository

[Zaunei]

As discussed in #342, I have implemented GitRepository as sourceRef. The implementation supports HTTPS cloning with and without authentication (currenly only Basic Auth).
I have been able to test it successfully with Github and Azure Devops repos. For the in-memory caching I had to adjusted the current implementation a little because it always returned a different path leading to cache misses. I may have misunderstood something there, but I would appreciate feedback on this in particular.

[coveralls]

Pull Request Test Coverage Report for Build 15569277031

Details

• 0 of 244 (0.0%) changed or added relevant lines in 3 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-3.4%) to 46.953%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/helm/chart/cache/cache.go	0	5	0.0%
internal/helm/chart/cache/inmemory.go	0	22	0.0%
internal/build/helm.go	0	217	0.0%

Files with Coverage Reduction	New Missed Lines	%
internal/helm/chart/cache/inmemory.go	1	0.0%
internal/helm/chart/dependency_manager.go	2	90.1%

Totals
Change from base Build 14901820554:	-3.4%
Covered Lines:	1649
Relevant Lines:	3512

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💛 - Coveralls