bt2: move common internal functions to their own files

This patch moves C code from `native_bt_component_class.i` to the new
`native_bt_bt2_objects.h` and `native_bt_log_and_append_error.h` files,
namely:

* bt_bt2_init_from_bt2(), which was bt_bt2_cc_init_from_bt2()
* bt_bt2_exit_handler(), which was bt_bt2_cc_exit_handler()
* The common logging and error appending functions.

This is just cleaner as those functions are not strictly related to the
component class interface.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ic9d7fed993b94b45f624e28e4c512b5ccab776bd
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1814
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>