Add option to enable commit signing

[jackleslie]

Context

At the moment, if you use this action and have the Require signed commits setting enabled for a protected branch, the PR generated can't be merged.

Example

Crowdin, a popular localisation management tool, has a similar action which opens a PR using a bot.

They allow you to pass a gpg_private_key as a config option, which will enable signed commits for the bot opening the PR. See options here.

[Andarist]

Sure, if you want to implement this - I would gladly review and merge a PR for this. For reference - this is what they do based on the gpg_private_key input:
https://github.com/crowdin/github-action/blob/b9b3c18f523d40926d75e48fc7fd2b6fdc23caee/entrypoint.sh#L174-L193

[jackleslie]

@Andarist opened a PR, let me know what you think!