feat: dependant toggles

[gardleopard]

This pr implements dependant feature toggles. The tests are specified in the client specifications Unleash/client-specification#63

[kwasniew]

Added some minor comments

[coveralls]

Pull Request Test Coverage Report for Build 6531319997

• 24 of 24 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 97.189%

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Totals
Change from base Build 6506745562:	0.1%
Covered Lines:	2524
Relevant Lines:	2597

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💛 - Coveralls

[sighphyre]

What happens to metrics? Parent features shouldn't get metrics incremented. Can we add a test for this?

[sighphyre]

G. I realise we want tests for metrics, this is quite tough to do with current structure. Yggdrasil already has those covered. If we want to move to that faster in lieu of tests here that's.... probably okay

[gardleopard]

What happens to metrics? Parent features shouldn't get metrics incremented. Can we add a test for this?

We are not touching the path of metrics here. Metrics behaviour is not tested in the sdk at all. If we want to have test for metrics behaviour I vote for having that as a separate PR.

[kwasniew]

@gardleopard the important part is that introducing dependent features shouldn't trigger metrics reported on the parent feature. If that's the case then we're fine. Tests can be added separately.