Support labeling files on the root partition other than `os_t`. e.g., `local_t`, `cache_t`.

**What I'd like:**
Have some defined path(s) on the root partition to use a label other than `os_t`. For example, label `/usr/cache-t/*` as `cache-t`, label `/usr/local-t/*` as `local-t`.

Why I want this feature?
I need to store some read-only files on the root partition. But those files are data files, say a db file. Or binaries that not supposed to executed by host or kernel. So `os_t` doesn't fit.

**Any alternatives you've considered:**