Rework providers e2e testing trigger

[eromanova]

Currently, our provider E2E testing is triggered by the test e2e PR label. However, there is no straightforward way to provide overrides for the testing configuration. There is a need to test more complex configurations, such as deploying two clusters with different templates on a single management cluster, testing the deployment of the N-1 template, and upgrading to the N template, among others.

I suggest the following changes:

Trigger provider E2E testing when the PR was commented starting with the /test-e2e trigger word followed by an optional testing configuration in a YAML format. I’m also considering combining both conditions: to trigger the provider testing on a PR, it should contain both the test e2e label and a comment containing /test-e2e. Since labels on a PR can only be added by our team (afaik, but I may be mistaken), this will help avoid unnecessary test runs triggered by someone else, not part of our team. We can keep triggering provider e2e testing on the existence of the test e2e label if there is no need to provide the testing config.

Other jobs: build and controller-e2etest may be triggered on each PR regardless of the test e2e label or /test-e2e comment present. This task is only about the adapting the github build workflow. Testing configuration support will be implemented in #641.