Implement the GcsFileSystem::OpenInputStream() functions and tests for them.

Reporter: Carlos O'Ryan / @coryan
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 #11436

_{Note: This issue was originally created as ARROW-14345. Please see the migration documentation for further details.}