Add fixed-length number byte order override

Add fixed-length number byte order override

This patch makes it possible to specify an immediate byte order (suffix
of encoding length) to encode a fixed-length number, overriding the
current byte order without changing it:

    !be
    11 22 33 44
    [0xaabbccdd : 32]
    [0xaabbccdd : 32le]
    [0xaabbccdd : 32]
    ff ff ff ff

Result:

    11 22 33 44
    aa bb cc dd
    dd cc bb aa
    aa bb cc dd
    ff ff ff ff

Change-Id: I3d0c9cb3f0f30ef2f74980bf9c63a93be3bdca64
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/normand/+/11040
Tested-by: jenkins <jenkins@lttng.org>