Make ListTables authorization consistent with GetTable/CreateTable:
1. ListTables authorization now evaluates policies instead of owner-only checks:
- For namespace listing: checks namespace policy AND bucket policy
- For bucket-wide listing: checks bucket policy
- Uses CanListTables permission framework
2. Remove owner-only filter in listTablesWithClient that prevented policy-based
sharing of tables. Authorization is now enforced at the handler level, so all
tables in the namespace/bucket are returned to authorized callers (who have
access either via ownership or policy).
3. Add flexible PolicyDocument.UnmarshalJSON to support both single-object and
array forms of Statement field:
- Handles: {"Statement": {...}}
- Handles: {"Statement": [{...}, {...}]}
- Improves AWS IAM compatibility
This ensures cross-account table listing works when delegated via bucket/namespace
policies, consistent with the authorization model for other operations.
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