Add `add_all_reviewers` boolean input

[mvlassis]

Is your feature request related to a problem? Please describe.
We are looking into incorporating this action in our team, and we want to automatically assign all reviewers of the original PRs on the backports. Unfortunately, the add_requested_reviewers option is not super useful in our case, since the requested reviewers have probably already reviewed (and approved), and thus do not show up when add_requested_reviewers is set to true.

Describe the solution you'd like
I think we can make use of the listReviews function and try to create a set of all people who have already reviewed. Upstream documentation on the function can be found here.

Describe alternatives you've considered
This input is supposed to be a superset with add_requested_reviewers since this is supposed to be a superset (requested + people who have already reviewed). There shouldn't be an issue if both inputs are set to true, but perhaps it could make sense to merge them into a add_reviewers input or similar, with non-boolean accepted values.

Additional context
There already is a similar issue at #2, but I'm not sure whether it refers to the exact same thing. #4 is supposed to close that issue (according to the PR description), but the issue is still open.

I can work on the implementation if it is deemed that the option makes sense :)

[korthout]

Thanks for writing it out so nicely @mvlassis! While actually a duplicate of #2, that issue was not clearly worded. So, I've closed it in favor of yours. It's clear what you are proposing.

I also like the name for the input: add_all_reviewers is quite clear I think. Make sure to clarify in the docs, that it includes both the requested_reviewers and the users that left a review. Might be good to point users to the docs of copy_requested_reviewers if they want the other behavior.

Regarding the interaction with add_author_as_reviewer and copy_requested_reviewers, I'd be happy if it would just add them on top, which is what the request reviewers API of GitHub does by default.

In other words, the following:

with:
  add_author_as_reviewer: true
  copy_requested_reviewers: true

On an original PR with:

author: @korthout
reviews: [{ name: @mvlassis }]
requested_reviewers: [@foobar]

Then the following users should get a review request for the backport PR:

@korthout 
@mvlassis 
@foobar

But these can be requested in the code separately:

1. For add_author_as_reviewer: Add @korthout
2. For add_all_reviewers: Add @mvlassis
3. For copy_requested_reviewers: Add @foobar

The result is the full set.

[korthout]

Happy to review a PR by the way 🚀

[mvlassis]

Created #578, I'm not sure how I can assign you as a reviewer so here's a small ping @korthout