* s3: list every bucket when auth is off
With no identities configured, every request may create, read and
write any bucket, but ListBuckets treated the caller as an
unauthenticated user and returned an empty list, so `aws s3 ls` showed
nothing. List all buckets in that mode, with prefix, max-buckets and
continuation tokens as before. Listing with auth on is unchanged.
* s3: cover the auth-on anonymous ListBuckets case
The no-auth listing change also moves the nil-identity gate; pin the
security-relevant direction that a request without an identity still
sees no buckets once auth enforcement is on.
* s3: verify ListBuckets continuation token reaches the next page
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Co-authored-by: user.email <chrislusf@users.noreply.github.com>
