[C++] Add scalar string join kernel

[asfimport]

Similar to Python's str.join

For example, if the separator were "-", this takes input:

ListArray<list<item: string>>
[
  ["foo","bar"],
  ["push","pop"]
]

and returns output:

Array<string>
[ 
  "foo-bar",
  "push-pop"
] 

Reporter: Maarten Breddels / @maartenbreddels
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [C++] String algorithm library for StringArray/BinaryArray (is a child of)
• [C++] Add variadic string join kernel (is related to)

PRs and other links:

• GitHub Pull Request #8990

_{Note: This issue was originally created as ARROW-10959. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 8990
#8990