An implementation for cache invalidations on Fastly has been added.
Specific paths can be invalided using Fastly's cached URL endpoint,
but this does not support wildcard invalidations.

Documentation on Fastly's invalidation API can be found here:
https://developer.fastly.com/reference/api/purging/

After uploading a new version of the database dump, the cached version
needs to be invalidated in CloudFront and Fastly.

When invalidating a CDN fails, we want to log a warning and continue
execution. This ensures that invalidations against all CDNs are
attempted.

An instance of the Fastly struct has been added to the `Environment` so
that it can be shared between jobs.

If the API token for Fastly is missing in the environment, skip
invalidating the cache there. This makes it possible to opt-in to the
new feature.