bt2: add bit array field class and field support

bt2: add bit array field class and field support

This patch adds bit array field class and field support to the Python
bindings.

A bit array field class has a `length` property (number of bits in the
array).

As of this patch, a bit array field is not a Python sequence. It could
become in the future however. The way to access a bit array field's
value is the `value_as_integer` property which simply wraps the
corresponding library functions.

A bit array field is not considered a numeric field. Its _spec_eq()
method only compares if the other object is also a bit array fields.

I did not implement the __bool__() operator because it will need to
satisfy the Python sequence protocol (length is not 0, which in fact is
always the case with a bit array field), should we decide as such in the
future. I implemented the __len__() operator however which is
straightforward.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I2e90e1444151bf5169a8df70c53664096cceb6c4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1918
Tested-by: jenkins <jenkins@lttng.org>