Use Redis backend to cache installation access tokens

Feature Request

Is your feature request related to a problem? Please describe.

When deploying a probot app as a cluster or a serverless deployment, new installation access tokens are being created even though the app created ones before that might still be valid. Probot does cache installation access tokens but currently on in memory. As we already support a Redis backend for throttling requests, we could reuse the same for caching tokens

Describe the solution you'd like

see https://github.com/octokit/auth-app.js/issues/117#issuecomment-663704870

Describe alternatives you've considered

Teachability, Documentation, Adoption, Migration Strategy

No breaking change. We should add a section to the docs about cluster/serverless deployments and why using Redis can help make it more efficient.