net: conn_mgr: Create separate connectivity implementation API.

Splits many definitions from conn_mgr_connectivity.h off into their own
header, conn_mgr_connectivity_impl.h

conn_mgr_connectivity.h now focuses solely on user/application-facing
APIs, and conn_mgr_connectivity_impl now contains all functions, utils,
definitions intended for use by connectivity implementations, including
conn_mgr_if_get_binding, which is now available for use by connectivity
implementations.

This lays the foundation for allowing connectivity implementations
easier and safer access to their internal state through internal APIs.

Signed-off-by: Georges Oates_Larsen <georges.larsen@nordicsemi.no>