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

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1. Reconstruct all comms after querying kernel #14468
   debt ipywidgets +2
   
2. Unable to regsiter comm target in extension host #14776
   bug debt partner +3