arrow

ARROW-12657: [C++] Adding String hex to numeric conversion

This PR adds the ability for the StringConverter to parse hex strings of the form:
0x123abc to an integer.
The strings must start with "0x" case insensitive.
The values ABCDEF are case insensitive.

Note that there was already a Hex Parsing function here:
https://github.com/apache/arrow/blob/master/cpp/src/arrow/util/string.cc#L76
Which was not really flexible enough for what was needed.
Not sure it it would be best to replace it with the new functionality added by this PR. I am open to suggestions

This work was originally done here #11064 but had to create a new PR due to a rebase issue

Closes #11161 from wmalpica/ARROW-12657_3

Lead-authored-by: William Malpica <16705032+wmalpica@users.noreply.github.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>