CommandHandler::ProcessCommandDataElement broken for group paths

[bzbarsky-apple]

Problem

CommandHandler::ProcessCommandDataElement does:

    err = commandPath.GetEndpointId(&endpointId);
    SuccessOrExit(err);

but this will fail for group paths. What should instead happen is that if there is no endpoint id we check for a group id (which is NOT in the command path; it's in the packet header, iirc), and then loop over all endpoints in the group doing the command on each endpoint.

Proposed Solution

Fix things so group commands work, like they used to work in the Ember code.

@yunhanw-google @erjiaqing

[yunhanw-google]

OK, we would handle it, we also need to remove group id from IM command path in MessageDef

[bzbarsky-apple]

@mrjerryjohns should probably confirm what the encoding side looks like before we start changing it.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[bzbarsky-apple]

Fixed in #13833.