fs_mgr: Add a method for creating one-off partition instances.

fastbootd needs to temporarily map and unmap logical partitions in order
to flash them. To make this easier, this change introduces fs_mgr
methods to create and destroy a single logical partition by name.

Bug: 79173901
Test: N/A
Change-Id: Ic085f2175c26a5edf5f18544012dcfe6d8088f09