[C++] Implement example string scalar kernel function to assist with string kernels buildout per ARROW-555

[asfimport]

I will write a patch to provide an example of creating a string-input string-output kernel for executing scalar-valued string functions

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• [C++] C++ array kernels framework and execution buildout (umbrella issue) (is a child of)
• [C++] String algorithm library for StringArray/BinaryArray (relates to)

PRs and other links:

• GitHub Pull Request #7278

_{Note: This issue was originally created as ARROW-8922. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 7278
#7278