[@vercel/blob] Restore the useCache option on get() (#1081)

useCache: false bypasses the CDN cache and serves the blob directly
from origin storage via the cache=0 query parameter (restored in the
CDN), guaranteeing the latest content. Defaults to true. This
un-deprecates the option that became a no-op in #1075.

The cache bypass is only sent for private blobs; the backend rejects
cache=0 on public stores, so get() quietly skips it there rather than
failing. Intentionally left out of the public docs.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>