checkout-repository, issue-repository, and ref inputs

[ianlewis]

• checkout-repository: the repo of the code being scanned.
• checkout-ref: the ref of the checked out code
• issue-repository: the repo where the issues are

This allows users to call the todo-issue-reopener from a repository that is different from the one being scanned.

Probably not a major use case as the repository scanned will almost always be the same. This probably also requires a GITHUB_TOKEN that has access to check out the checkout-repository, and write issues to the issue-repository.