Configure auto merge for dependabot via config file

[nostadt]

Currently it's possible to create auto merge via a respective CI job. https://docs.github.com/en/code-security/dependabot/working-with-dependabot/automating-dependabot-with-github-actions#enable-auto-merge-on-a-pull-request

I believe it would be helpful if I can toggle it with one config line in the dependabot.yml

auto_merge: true

Idea: This can be configured for each package manager individually.