* s3: deny anonymous requests when the identity config loads no identities
Naming a config file is the operator asking for authentication. A file that
yields no identity - an unpopulated secret mount, or a mistyped top-level key
the proto parser silently drops - left the gateway open to every anonymous
caller: ListBuckets returned 200, and anonymous PUT could create buckets and
write objects.
* s3: name the unknown top-level keys in an identity config
The proto parser discards what it does not recognise, so a mistyped
"identites" loads as an empty config. Naming the dropped keys at startup turns
the resulting lockout into a one-line diagnosis.
* s3: isolate the auth-enforcement tests from AWS environment credentials
* s3: use a singular "identity" as the unrecognised-key example
Codespell rejects the misspelling the example used.
* s3: cover the empty identity config alongside the unrecognised key
* s3: cover a config file whose body is an empty object
