Remove dependence on boskos and test-infra

[palnabarun]

Filing this issue to remove dependency of kubetest2 on boskos and in turn test-infra.

Thoughts from @BenTheElder on #207 (comment):

The best answer is probably copying in our own boskos client like @munnerz did in test-infra, boskos client is pretty simple and stable, and the API is not really changing.

[palnabarun]

The prior art by @munnerz in kubernetes/test-infra#26956 copies relevant parts of boskos to test-infra and eliminates the circular dependency between boskos and test-infra.

However, the problem we are dealing with here is that boskos client depends on test-infra for test-infra/prow/config/secret. We need to find a way to resolve that as well.

Otherwise, merely copying boskos client code here will still have dependency on test-infra.

[BenTheElder]

I think given our current discussion the correct answer for now is for boskos to stop depending on the secret agent xref kubernetes/test-infra#26989 (comment)

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/lifecycle stale

[BenTheElder]

/lifecycle frozen

[kumiDa]

/sig testing

[upodroid]

/assign