Storage buckets: add more plumbing for CacheStorage

Since CacheStorage requires some extra params related to
CrossOriginEmbedderPolicy, BucketContext is no longer a struct and
is replaced by an interface which worker and frame hosts implement.
This interface should encapsulate all the info required to service
buckets API requests.

For now the returned CacheStorage object will always be the one for
the default bucket.

Bug: 1264128
Change-Id: I2f770e74134af4eaaafb8471dc62a8d2b219b5fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3838637


Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1046022}