Implement mechanism for a node to request connection to a micro service.

[conradgrobler]

Example: trusted time might be provided as a psuedo node, rather than a host function. A wasm node should be able to request a channel and then use the channel to request the trusted time. The channel should block requests once the node has seen any secret data, as time request make external network requests which could be used as a side channel to leak data.

[daviddrysdale]

We now have pseudo-Nodes that allow:

• connection to an external gRPC service (gRPC client pseudo-Node (C++ runtime) #640)
• connection to a Roughtime client (Add Roughtime client pseudo-Node #845)

However, still need to check that information flow control works correctly with these pseudo-Nodes when it gets fully implemented (#630).