feat: sparse writes with range_upload, zero-download write path #957

Job	Run time
Lint & Unit Tests	1m 30s
xfstests (filesystem exerciser)	11m 39s
POSIX Compliance (pjdfstest)	1m 43s
fsx (data integrity)	1m 6s
Smoke Tests (FUSE + NFS)	4m 17s
Benchmarks	2m 15s
	22m 30s

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