fix(controller): make pull request-based promotions honor cert ignore option

[krancour]

Follow up to #2158, #2145, #1274

The last remaining difficulty I have been able to detect with PR-based promotions to repositories in self-hosted GitHub Enterprise or GitLab is that the insecureSkipTLSVerify option available at the pull request level is not honored by the clients used by the relevant GitProviders.

This PR fixes that.

And with this PR, I can also confirm Kargo working e2e with an actual instance of GHE Server.

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 53 lines in your changes missing coverage. Please review.

Project coverage is 46.32%. Comparing base (f211d9e) to head (6442b1d).

Files	Patch %	Lines
internal/gitprovider/gitlab/gitlab.go	0.00%	22 Missing :warning:
internal/gitprovider/github/github.go	0.00%	14 Missing :warning:
internal/controller/promotion/pullrequest.go	0.00%	8 Missing :warning:
internal/gitprovider/registry.go	0.00%	8 Missing :warning:
internal/controller/promotion/git.go	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2170 +/- ## ========================================== - Coverage 46.37% 46.32% -0.05% ========================================== Files 242 242 Lines 16737 16754 +17 ========================================== Hits 7761 7761 - Misses 8606 8623 +17 Partials 370 370 ```

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