Add option to customize the username

[yeikel]

I am currently running dependabot script in a private environment and I would like to leverage this Action

The main challenge at the moment is this action expects the sender to be an specific user that does not meet my environment:

https://github.com/ahmadnassri/action-dependabot-auto-merge/blob/master/action/index.js#L21-L24

Would you consider adding a parameter to this action to support this use case? We can default it to dependabot[bot] and dependabot-preview[bot]

I am willing to submit a PR given that you agree with this change

[ahmadnassri]

for sure, a PR is most welcome