Split out store-open.hh and store-registration.hh

The existing header is a bit too big. Now the following use-cases are
separated, and get their own headers:

- Using or implementing an arbitrary store: remaining `store-api.hh`

  This is closer to just being about the `Store` (and `StoreConfig`)
  classes, as one would expect.

- Opening a store from a textual description: `store-open.hh`

  Opening an aribtrary store implementation like this requires some sort
  of store registration mechanism to exists, but the caller doesn't need
  to know how it works. This just exposes the functions which use such a
  mechanism, without exposing the mechanism itself

- Registering a store implementation: `store-registration.hh`

  This requires understanding how the mechanism actually works, and the
  mechanism in question involves templated machinery in headers we
  rather not expose to things that don't need it, as it would slow down
  compilation for no reason.

tests/functional/test-libstoreconsumer/main.cc

@@ -1,5 +1,5 @@
 #include "nix/store/globals.hh"
-#include "nix/store/store-api.hh"
+#include "nix/store/store-open.hh"
 #include "nix/store/build-result.hh"
 #include <iostream>