3PH part 1: Prepare for 3PH support

[Semisol]

This PR does not implement any 3PH related functionality, except preparations:

• A new Network3PH interface has been added to be implemented by networks supporting 3PH. (this could be named better)
• 3PH-related methods have been moved out of Network for networks that do not support it.
• Schemas have been updated to match the capnproto/capnproto repository.

To be done:

• Initial 3PH support, with forwarding and Resolve with 3rd party capabilities
• Embargo support for 3PH
• An extension to the API that allows tail calls (This is not going to be a priority for me, as it is only useful in rare conditions)

[lthibault]

Thanks for the contribution!
I think there's a dependency issue causing CI to fail. I had a look in the meantime, and it LGTM overall. Let's get CI passing and then merge it in.

[Semisol]

The automatic schema generator seems to not be able to handle the case of schemas in directories, with compat/ as an example. I'll push an update that fixes this.