client_test.go

What
---
- Update the `client.HttpClient` fields to have `http.Client` and
  `retryablehttp.Client`, one of them will be `nil` depending on the
  feature flag toggle.
- Create new method `newRetryableRequest` which will create a
  `retryablehttp.Request` and use that if the
  `FF_GITLAB_SHELL_RETRYABLE_HTTP` feature flag is turned on.
- Add checks for `FF_GITLAB_SHELL_RETRYABLE_HTTP` everywhere we use the
  http client to use the `retryablehttp.Client` or the default
  `http.Client`
- New job `tests-integration-retryableHttp` to run the integraiton tests
  with the new retryablehttp client. We didn't update go tests because
  some assertions are different and will break table driven tests.

Why
---
As discussed in
https://gitlab.com/gitlab-org/gitlab-shell/-/merge_requests/703#note_1229645097
we want to put the client behind a feature flag, not just the retry
logic. This does bring extra risk for accessing a `nil` field but there
should be checks everytime we access `RetryableHTTP` and `HTTPClient`.

Reference: https://gitlab.com/gitlab-com/gl-infra/production/-/issues/7979


Signed-off-by: Steve Azzopardi <sazzopardi@gitlab.com>