Commit bench output through the Github API for verification's sake

[joao-paulo-parity]

See #15 for motivation

See https://github.com/joao-paulo-parity/bot-playground/pull/10 for how it works. The functionality won't be changed, only the bot commits will be verified now (https://github.com/joao-paulo-parity/bot-playground/pull/10/commits/941b138acdd7645e97f453f31e3f12833f060a65).

This approach of committing through the API for bot verification is based on

• https://github.community/t/how-to-properly-gpg-sign-github-app-bot-created-commits/131364/2
• https://gist.github.com/harlantwood/2935203

I did not and will not try to "improve" the code stylistically in this MR. I'm only interested in getting the verification working for unblocking the CLA.

closes #15

Notes

• Needs "Contents" Github App permission for this to work
• The scope is done but it's draft because I have not confirmed if it works (permissions might be missing still)

[joao-paulo-parity]

This did not work out because of https://github.community/t/creating-a-tree-fails-with-must-supply-a-valid-tree-mode/178438/2. I am not sure if it's a viable solution because we'd be locked in a specific file mode through that API.

Using the API makes the whole process only slightly (even that's arguable) more sound because, even though the resulting commits get verified, they can still be impersonated if the bot's credentials get stolen. Seems like it's not worth pursuing this further, at least right now.