[C++] Consolidate random string generators for use in benchmarks and unittests

[asfimport]

This was discussed in here:

#3721

For testing/benchmarking dictionary encoding its useful to control the number of repeated values and it would also be good to optionally include null values.  The ability to provide a custom alphabet would be handy for generating strings with unicode characters.

 

Also note that a simple PRNG should be used as the group has observed performance trouble with Mersenne Twister.

Reporter: Hatem Helal / @hatemhelal

_{Note: This issue was originally created as ARROW-4661. Please see the migration documentation for further details.}

[asfimport]

Francois Saint-Jacques / @fsaintjacques:
Feel free to extend RandomArrayGenerator in testing/random.h, I'd love to see the value distribution given as an option struct in the construct instead of having min/max arguments.

[asfimport]

Hatem Helal / @hatemhelal:
I made a first pass at this to unblock testing for ARROW-3769:

#3721

I'd like to use this issue to work on the option struct as suggested by @fsaintjacques.

[github-actions]

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