Allow for OpenRPC message components to be shared between comms #7299
Description
In thinking about #7114 -- we need to allow for some amount of stateless data querying capability in the variables service. Currently all of the logic that exists for this in the runtime comms layer exists in the data explorer comm, but we could reuse many of these data structures to reduce duplication and improve maintainability.
I was thinking that we could define a common.json file in positron/comms with a set of OpenRPC schemas that can be referenced from any of the other comm-specific service definitions -- something like "$ref": "common.json#/data_explorer/..." compared with the current #/components/schemas/... -- see https://github.com/Mermade/open-rpc-spec/blob/master/README.md#reference-object). This would sharing schemas more easily.