Bluetooth: Mesh: Isolate cryptographic material

This is a major refactoring of the handling of the cryptographic
material of both the network and transport layers. The aim is to
encapsulate the key object manipulation, and improve overall modularity.

Pulls Applications and Subnets out of the bt_mesh and into separate
modules, with static storage types on the data. This has several
side-effects:
- The Config Server no longer operates directly on the bt_mesh.subs and
  bt_mesh.apps lists, but goes through a public configuration interface,
  following the pattern set in #27908.
- All iteration through the keys is done through iteration APIs
- Key resolution on RX and TX is centralized.
- Changes to the keys triggers events the other modules can register
  handlers for.
- Friendship credentials are stored in the lpn and friend structures.

Part of #27842.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>