lib: have dedicated "dynamic array FC with/without length field" types

This is to match what is already done for option and variant field
classes: you can know with their type enumerator whether or not they
have a selector and which type it is.

Now, when you pass `NULL` as the length FC to
bt_field_class_array_dynamic_create(), you get a FC with the type
`BT_FIELD_CLASS_TYPE_DYNAMIC_ARRAY_WITHOUT_LENGTH_FIELD`. Otherwise
it is `BT_FIELD_CLASS_TYPE_DYNAMIC_ARRAY_WITH_LENGTH_FIELD`.

Now you can only call
bt_field_class_array_dynamic_with_length_field_borrow_length_field_path_const()
with a FC having the type
`BT_FIELD_CLASS_TYPE_DYNAMIC_ARRAY_WITH_LENGTH_FIELD`. The way to know
whether or not the dynamic array FC has a length field path is to check
its type.

In the Python bindings, there are dedicated
`_DynamicArrayWithLengthFieldFieldClass` and
`_DynamicArrayWithLengthFieldFieldClassConst` classes to match the
library changes. The `length_field_path` property is only available in
those.

A `sink.text.details` component now writes

    Dynamic array (no length field)

and

    Dynamic array (with length field)

depending on the dynamic array FC's type.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ibdc0ef1110bf05d517b241ba4f6a9a631ade8a0c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2169
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>