Key Cache Invalidation issue

The runtime caches keys the first time they are accessed, and relies on file change notifications to refresh the cache. For File storage keys those change notifications happen as part of modifying the key files, and for Blob storage keys we use a sentinel file that we watch.

However, in environments without a shared file system to use for notifications (e.g. Linux Consumption) this mechanism is broken. We need to come up with a replacement mechanism.

**For the keys admin APIs, I think we should bypass cache completely.** The cache is only needed for runtime function invocation performance.

No files or tests are named. Start by tracing the runtime key cache and the key admin APIs described in the issue; define a replacement for file-change notifications on environments without a shared file system, while ensuring admin APIs bypass the cache.

Written by the indexing model from the issue text.