Add telemetry for when Copilot is assigned to an issue

[alexr00]

[alexr00]

To verify:

• Install the latest pre-release of GitHub Pull Requests
• Open your clone of a GitHub repo
• Command "Developer: set log level" and chose trace for everything
• Command "Developer: Show Telemetry"
• From the Issues view, open an issue description
• Assign the issue to Copilot
• Verify that you see pr.assignCopilot in the Telemetry output.