Add option to update PR on reuse

[grayside]

Current Behavior

When using the force: true option to reuse a Pull Request with an updated code suggestion, the existing Pull Request carries the new commit but changes to the title and description on the PR are not processed. (https://github.com/googleapis/code-suggester/blob/main/src/github/open-pull-request.ts#L50)

Requested Behavior

I would like an option such as update-pr-on-reuse so the title and description from my GitHub Workflow is used to update the PR.

Why

I prefer generated PRs to have detailed context about the inputs that went into the code change, and without updating the PR description, this context is out of date with the next commit on the branch. You can see an example of what I hope to enable with this feature at https://github.com/googleapis/google-cloudevents-go/blob/521da225b7e85d6901bdc316b9542d1060aad29e/.github/workflows/generate.yml#L96.