bt2: rename `_MessageIteratorConfiguration.can_seek_forward()`

bt2: rename `_MessageIteratorConfiguration.can_seek_forward()`

This method's name suggests that it's public and can be called by the
user, but that's not the case, since it is overwritten by the write-only
property just below.  Example:

      File
    "/home/smarchi/build/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/message_iterator.py",
    line 142, in _bt_init_from_native
        self.__init__(config, self_output_port)
      File "/home/smarchi/src/babeltrace/test.py", line 11, in __init__
        config.can_seek_forward(False)
        ^^^^^^^^^^^^^^^^^^^^^^^
    AttributeError: property 'can_seek_forward' of '_MessageIteratorConfiguration'
    object has no getter

Rename it to something that's more obviously not intended for the user.

Change-Id: Ibef9cd8297569e2d4daea01ede00f3f0735abd90
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13387
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>