Merge core.json into operations, and add case origins and S3 checksums

[tiptenbrink]

Summary

• Case origins. A case may declare origin: observed (the default, left out), transient (the service sends the response, but not on demand, such as a 503 under load) or constructed (no service is known to send it). The grader grades every origin alike. 37 cases carry a non-default origin.
• operations/s3/list-encoding-ignored, a constructed case: a service that ignores encoding-type=url sends raw keys and no EncodingType. The keys hold a % that begins no escape, which reads as text.
• core.json is merged into operations.json.
  • 42 core cases were already covered by an operations case and are dropped. So are six that checked only an error kind beside the status and code, and four binary-body cases that the gzip cases already round-trip.
  • The other 25 moved under operations names, such as operations/s3/get-xml-looking, operations/s3/get-version and operations/azure/list-pagination.
  • Three stay as the grader's own test fixture, tests/cases.json, so the grader's tests no longer depend on the corpus.
• S3 checksums.
  • put-with-crc32 sends one additional checksum.
  • put-two-checksums expects S3's 400 InvalidRequest for two x-amz-checksum- headers, and permits a local refusal over checksums.
  • list-checksum-algorithm has list entries report checksum_algorithm as one name. S3's API model makes ChecksumAlgorithm a list, but S3 keeps one checksum per object.

Every moved and new case was verified live against S3 and Azure, except list-encoding-ignored, which S3 cannot be made to send.

For clients

• Grading core.json stops working. Its cases now live in operations.json under new ids, so an expected-unsupported list that names core ids needs updating.
• An adapter that leaves out checksum_algorithm on list entries gets list-checksum-algorithm wrong, unless it declares the field in unsupported_fields.

Test plan

• cargo test --locked, cargo clippy --locked --all-targets -- -D warnings and cargo fmt --check pass.
• object-tests validate passes for cases/operations.json (713 cases) and cases/s3-express.json (164 cases).