[C++] Add is{alnum,alpha,...} kernels for strings

[asfimport]

A good list of kernels to have would str->bool kernels, similar to:

https://docs.python.org/3/library/stdtypes.html#str.isalnum and friends.

I think all but isidentifier make sense to have. The semantics of the Python functions seem quite reasonable to have in Arrow, but maybe others can provide feedback if this is a complete/reasonable list to have or not.

I am not sure if we need more (or less) functions, or if we want more atomic functions, e.g. test for membership in Unicode categories. 

Reporter: Maarten Breddels / @maartenbreddels
Assignee: Maarten Breddels / @maartenbreddels

Related issues:

• [C++] String algorithm library for StringArray/BinaryArray (is a child of)

PRs and other links:

• GitHub Pull Request #7656

_{Note: This issue was originally created as ARROW-9268. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
Started to build the foundation for str->bool in #7593.

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 7656
#7656