ci: enable Automatic Merge of Approved Pull Requests

[phracek]

Add automation that will merge approved PRs with passed CI checks automatically.

It uses the following GitHub Actions:

• gather-pull-request-metadata - to gather important data about PRs (used in following actions)
• pull-request-validator - to check if PR is approved and has passed CI checks and more
• auto-merge - to merge approved PRs
• issue-commentator - to comment on PR with status of PR and merge status

Here are some examples of use from the systemd source-git. Please notice the Pull Request validation and Auto Merge sections:

• (RHEL-30372) Drop /dev test in test-mountpoint-util redhat-plumbers/systemd-rhel9#281 (comment)
• (RHEL-25529) Backport rhel-net-naming-sysattrs related code to RHEL 9.2.0 redhat-plumbers/systemd-rhel9#277 (comment)
• Update README.md jamacku/nginx-container#5 (comment)
  Similar PR is here: Enable Automatic Merge of approved Pull Requests nginx-container#306
  /cc @jamacku

[jamacku]

LGTM, But I would suggest you to create a composite GitHub action for sclorg that would call:

• redhat-plumbers-in-action/gather-pull-request-metadata@v1
• redhat-plumbers-in-action/pull-request-validator@v2
• redhat-plumbers-in-action/auto-merge@v2
• redhat-plumbers-in-action/issue-commentator@v1

It should make the maintenance of automation easier since it should be in one place.
Here is an example for inspiration: https://github.com/redhat-plumbers-in-action/source-git-automation/blob/main/action.yml

[phracek]

I agree, but let's test it on two containers and later on we will decide whether we will use it in all repositories.