Enforce the CLA requirement by using an action to reduce overhead and unecessary information

[jvpichowski]

For now, you need to always check a box in the PR commentary to say that you have signed the CLA.
This has several disadvantages:

1. You need to do this again for every PR. This is an unnecessary repetitive task.
2. The information given is not interesting for the PR because it reveals no specific content.
3. The information is not stored centrally. So if you want to get a list, of all users who have signed the CLA you need to check all closed pull requests of all projects.

I propose an approach using the CLA-Assistant action.
This one is developed by SAP and certainly verified by usage.
You only need to verify on the first PR that you have signed and submitted the CLA.
You get immediate information on what steps to do, to submit your PR.
And all users are stored in a private repository to keep a centralized list, which could be easily checked to the local list of CLAs in the WüSpace administration.

I will make a PR if you like this idea.