meta: add Af2Desc metadata storer for CDM SDFS AF2 DESC attribute

[riaarora-boop]

Summary:

Adds Af2Desc, a new MetadataStorer implementation for CDM SDFS. It packs must-store S3 object metadata (etag, content-type, content-encoding, user metadata, etc.) into the single sdfs_af2_file_desc xattr so it rides the AF2 partition snapshot alongside the object data.

Key design points:

• Only the sdfs_af2_file_desc key works on SDFS FUSE — generic xattr keys are rejected and getxattr is unimplemented. Af2Desc packs all metadata into one JSON blob under this key.
• Serves reads from a write-through in-process cache since SDFS cannot read xattrs back after write. Cold-cache misses return ErrNoSuchKey, which callers already handle as "empty."
• Non-allowlisted keys are accepted-and-dropped so the blob stays within the MJF value-size cap.

Exposes --af2-desc and --af2-desc-max-bytes flags on the posix subcommand. Mutually exclusive with --sidecar and --nometa.

Test Plan:

• go test ./backend/meta/... passes (unit tests cover store/retrieve/delete/rename/cache invalidation on a normal xattr-capable filesystem)
• Verified on SDFS on-cluster: setxattr with raw sdfs_af2_file_desc key succeeds; with user. prefix returns ENOTSUP

JIRA Issues:

CDM-557243 (subtask of CDM-557238)

Revert Plan:

Revert this PR. --af2-desc defaults to false so existing deployments using xattr or sidecar are unaffected.