RFE: Implement automatic approve

[markuslackner]

Problem statement.
At the moment the service will do nothing after opening the pull request. It would be a great enhancement in usability if the service approves the pullrequest automatically. This is dependent on multiple prerequisites:

• Which github/gitlab/... edition is used? e.g. you need Branch protection for github to guarantee the ci pipeline to be successful.
  Additionally the "Merge if successful" is only available with Branch protection. For outer git Providers this must also be checked.
• The auto approve must be configurable by the user (e.g. no auto approve to production)

Possible solution.
2 new configuration options:

• auto-approve: Approves the pull request
• auto-merge: Sets automatic merge if all checks are successful

=> If the used git provider or the repository configuration does not support the needed features an error will be shown.