Bluetooth: shell: Register callbacks dynamically

Bluetooth callbacks are now registered dynamically again. When Bluetooth
callbacks were registered statically, they could be invoked before the
Bluetooth shell (ctx_shell) is initialized. The callbacks try to
shell_print(ctx_shell, ...), which results in a fatal error if ctx_shell
is undefined.

Fixes #40881.

Signed-off-by: Andreas Rudolf <andreas.rudolf@husqvarnagroup.com>