Optimise GCSFilesystem rename implementation

[kennknowles]

GCSFileSystem implements a rename() with a copy and delete operation.

However, GCS has an Objects: rewrite which looks like it would be a metadata operation only and therefore has the potential to be much quicker.

Once BEAM-5036 is fixed IOs that write files will make use of a rename operation.

Imported from Jira BEAM-5429. Original Jira may contain additional context.
Reported by: timrobertson100.

[Abacn]

.close-issue not_planned
Function already there. Outdated.