bindings: Narrow uniffi interface

[cdecker]

This is the narrow uniffi interface that exposes a TlsConfig, a Scheduler and
a Signer class with its full API. The Node on the other hand does not have
all its methods, rather it has a constructor and a call method that takes
the method path, and the request payload encoded as protobuf payload, and it
returns the protobuf result. This allows us to not have to generate the method
signatures in IDL from msggen, but also pushes more of the responsibility to
the developer. On top of this we will later build the wide interface, that
uses msggen to generate the IDL file, and exposing all the methods natively.