BP5/XRootD: identity/version/byte-order checks on remote reads

[eisenhauer]

This PR is in some sense a roll-up of new information that may be (but likely currently isn't) part of an HTTPS request. This items include :

• the UUID of the file requested to ensure it matches what the remote has (omitted for UUID=0, which is true for all files except BP5 files newer than PR BP5: add per-file id to index header #5057).
• the byte order of the requesting client (omitted if little-endian).
• a request-version number (omitted if zero, which it currently is, but here to handle the inevitable changes).

[eisenhauer]

The SPIN-deployed server is now running with these changes and it properly rejects queries to files with the right path but the wrong metadata UUID. This, plus the in-CI XRootD tests (which will exercise the matching-UUID path). Should be sufficient testing for now. We've added a version number to the request to cover the circumstance where we change the format and want to throw errors on server/request mismatch (no changes anticipated yet). We've also got the client byte-order in the request, should the world get more heterogeneous again...