controller: introduce reconcileResult for MantleBackupReconciler #1373

Job	Run time
build (1.35.1, 3Mi, full-backup)	34m 53s
build (1.35.1, 3Mi, change-to-primary)	30m 7s
build (1.35.1, 3Mi, !//)	47m 44s
build (1.34.4, 3Mi, full-backup)	36m 28s
build (1.35.1, 10Mi, various-transfer-part-size)	22m 28s
build (1.35.1, 3Mi, lock)	25m 31s
build (1.35.1, 3Mi, backup-failure)	34m 15s
build (1.35.1, 3Mi, change-to-standalone)	24m 29s
build (1.35.1, 3Mi, incr-backup)	38m 4s
build (1.35.1, 200Gi, various-transfer-part-size)	24m 28s
build (1.35.1, 3Mi, change-to-secondary)	31m 14s
	5h 49m 41s

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