Introduce soong_filesystem_creator module type

The module generates soong-built filesystem modules. This module creates
the system partition submodule, and will create other partition
filesystem submodules in the future.

Note that not all properties of the system submodule is currently set;
some properties like `deps` are not currently set.

Test: m nothing --no-skip-soong-tests
Bug: 369682648
Change-Id: I671a9b6a61e5453117f604394191243c5ae047d9