Add `DELETE` compatibility to S3/HTTP backends

[jhiemstrawisc]

Pelican sneakily allows deletions through the mostly-undocumented pelican object delete command. When this command is issued against a namespace backed by a POSIX origin, an HTTP DELETE is sent to the Origin to remove the object.

However, @brianaydemir recently tried this against an S3-backed Origin and encountered a "function not implemented error:

 ofs_remove: <REDACTED USER/IP> Unable to remove <REDACTED OBJECT NAME>; function not implemented" daemon=xrootd.origin

It'd be nice to support this.