Create event loop when client constructor needs one

[Mirochill]

Fixes #18.

Summary:

• Add a small event-loop helper for the default CPGQLSClient constructor path.
• Preserve supplied event_loop instances, while creating and installing a new loop when the current thread has none.
• Add a regression covering construction with no current event loop.

Validation:

• git diff --check HEAD~1..HEAD
• Not run locally: pytest suite, package build, import/runtime check