Explore supporting Comms messaging on code execution

[DonJayamanne]

Current proposal

• Ensure Comms API only works within the context of a pending request execution
  • I.e. 3rd party makes a request to execute code via the kernel API
  • & comms is valid only until this execution request has not resolved
  • Once execution completes, the comms API will not be available.

### Tasks
- [ ] https://github.com/microsoft/vscode-jupyter/issues/14468
- [ ] https://github.com/microsoft/vscode-jupyter/issues/14776