Add auto-merge

[Ben10k]

Description

Solves issue #155

This PR:

• Enable automated merges on promotion PRs, by configuration in telefonistka.yaml
• Enable automated merges on version bumps telefonistka bump-regex and telefonistka bump-overwrite
• Refactor bump-regex and bump-overwrite to use the same method of getting credentials as in promotion flow
• Add documentation for new configuration options

All new functionality is tested manually

Type of Change

• Bug Fix
• New Feature
• Breaking Change
• Refactor
• Documentation
• Other (please describe)

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass

[Oded-B]

I'll try to take a look at this in the next day or two, looks very promising in first glance, thanks!