feat: use labels for test execution on PRs to main

[awownysz-splunk]

Context

So far, PRs to main by default have all the tests enabled. Applying labels to enable particular tests is available only for PRs that are not to main.
Our team doesn't use develop branch, thus we need to be able to skip some tests on PRs to main - to get feedback quickly.

Summary of changes

• This PR makes it possible to use labels for enabling/disabling tests also on PRs to main.
• apart from test labels (execute_knowledge, etc.), use_labels label has to be applied to the PR before (re)opening it to make test labels work - this is to ensure that execution of all tests is still the default option for PRs to main

Sample CI/CD runs for PR

PR: https://github.com/splunk/splunk-add-on-for-microsoft-cloud-services/pull/993
Workflow runs:

• with use_labels label: https://github.com/splunk/splunk-add-on-for-microsoft-cloud-services/actions/runs/7338668760
• with use_labels and execute_knowledge labels: https://github.com/splunk/splunk-add-on-for-microsoft-cloud-services/actions/runs/7338999846/job/19982527979
• with no labels: https://github.com/splunk/splunk-add-on-for-microsoft-cloud-services/actions/runs/7339852310/job/19984825125

[srv-rr-github-token]

🎉 This PR is included in version 4.11.0 🎉

The release is available on GitHub release

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