Merge cockroachdb#162034

162034: restore: pull restoration flow creation out of createImportingDescriptors r=dt a=msbutler

Previously createImportingDescriptors created the restoring descriptors _and_ planned the restoration flows. This patch pulls the restorationg flow logic out of createImportingDescriptors into a seperate helper, createRestoreFlows.

Epic: none

Release note: none

Co-authored-by: Michael Butler <butler@cockroachlabs.com>

Author: craig[bot]