Add ext full backup (#1281)

* initial

Signed-off-by: hillium <[email protected]>

* added hint of prefix

Signed-off-by: hillium <[email protected]>

* added force flush

Signed-off-by: hillium <[email protected]>

* make result repeated

Signed-off-by: hillium <[email protected]>

* make full backup a reference

Signed-off-by: hillium <[email protected]>

* Revert "added force flush"

This reverts commit a51d008.

Signed-off-by: hillium <[email protected]>

* rollback irraltive changes

Signed-off-by: hillium <[email protected]>

* added more comments

Signed-off-by: hillium <[email protected]>

* address comments

Signed-off-by: hillium <[email protected]>

* rename extra full backup to ingested SSTs

Signed-off-by: hillium <[email protected]>

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Signed-off-by: hillium <[email protected]>
Signed-off-by: hillium <[email protected]>