[Libomptarget] Factor functions out of 'Plugin' interface (#86528)

Summary:
This patch factors common functions out of the `Plugin` interface prior
to its removal in a future patch. This simply temporarily renames it to
`PluginTy` so that we could re-use `Plugin::check` internally as this
needs to be defined statically per plugin now. We can refactor this
later.

The future patch will delete `PluginTy` and `PluginTy::get` entirely.
This simply tries to minimize a few changes to make it easier to land.