Support Ansible Galaxy

Ansible Galaxy allows for a `requirements.yml`:
https://docs.ansible.com/ansible/2.9/galaxy/user_guide.html#install-multiple-collections-with-a-requirements-file

It would be nice if Dependabot could scan this and check against https://galaxy.ansible.com/search

Start with the requirements.yml format in the linked Ansible Galaxy user guide, then inspect the Galaxy search page to understand how collections and available versions are identified. Done means Dependabot can scan requirements.yml and check its Ansible Galaxy dependencies for updates.

Written by the indexing model from the issue text.