[C++] Add google-cloud-cpp to ThirdpartyToolchain

[asfimport]

This is a requirement to be able to make progress on ARROW-1231

Reporter: Wes McKinney / @wesm
Assignee: Carlos O'Ryan / @coryan

Related issues:

• [C++] Add filesystem / IO implementation for Google Cloud Storage (is a child of)

PRs and other links:

• GitHub Pull Request #11268
• GitHub Pull Request #8757

_{Note: This issue was originally created as ARROW-8147. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 11268
#11268