fs_mgr: Add a helper for creating one-off dynamic partitions.

This will be used by gsid to invoke mkfs.ext4 on the userdata_gsi
partition. Since the extents are not located on the super partition, we
need a helper method that takes in an LpMetadata.

Bug: 121210348
Test: manual test
Change-Id: I00467ace8a745fb0c0d130babfda1a2d5d97c208