MINOR: sample: add the "be_sort" converter

MINOR: sample: add the "be_sort" converter

This converter sorts in ascending numerical order the big-endian unsigned
integers of <elem_size> bytes stored in a binary sample. <elem_size> must
be 1, 2, 4 or 8, and the input length must be a multiple of <elem_size>,
otherwise the conversion fails.

Since the elements are unsigned and stored in network byte order, comparing
their bytes starting from the most significant one gives the numerical
order, so a simple memcmp() over the element width is enough and no byte
swapping is needed.

It is mainly intended to normalize a list of binary values whose order may
vary from one client to another before hashing it, for instance the cipher
list or the extension list of a TLS client hello:

    http-request set-var(txn.ciphers) \
        ssl_fc_cipherlist_bin(1),be_sort(2),xxh64

Documentation was updated.