bt2: add boolean field class and field support

bt2: add boolean field class and field support

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

A boolean field class has no properties.

A boolean field is a numeric field (like Python's `bool` is a numeric
type). The `BoolFieldTestCase` test case inherits `_TestNumericField`,
making sure the field behaves more or less like the Python `bool` type.
`BoolFieldTestCase` is very similar to `BoolValueTestCase`.

Just like `BoolValue` objects, you can set the value of a `BoolField`
object from a `bool` or `BoolField` object.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ib6d0723d69709b7f3b459ba173eb7cbea5d8c435
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1896
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>