[C++] Add API to parse URI query strings

[asfimport]

We already expose an API to parse URIs, but it doesn't handle query strings. Parsing query strings is required for ARROW-6720.

Reporter: Antoine Pitrou / @pitrou
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #5770

_{Note: This issue was originally created as ARROW-7057. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
Issue resolved by pull request 5770
#5770