Bluetooth: conn: Start connected-only grouping

Start a group of functions guarded by CONFIG_BT_CONN
that are for connected-only functions. This is a
preparation step for allowing compilation and usage of
conn.c for ISO broadcast-only builds.

Moving all connected-only functionality into a single
group will also allow a possible move of all those
functions into an connected (ACL) only file.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>