lib: move `bt_value` structures to `value-internal.h`

lib: move `bt_value` structures to `value-internal.h`

It's unusual to have internal structures in the C file. To remain
consistent with the rest of the library, put them in the internal
header.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I5a946004a172560acecab1b182214293f61f02fd
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1255
CI-Build: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Tested-by: jenkins