feat: new crate scion-grpc with setup for sciond

[mlegner]

This adds protobuf definitions and gRPC generation for the sciond communication. This is largely copied from https://github.com/MystenLabs/scion-hp-market/tree/main/crates/scion-grpc (partially from https://github.com/scionproto/scion, partially authored by @jpcsmith), but I left out all the other protobuf definitions as we probably won't need them.

The PR also updates other project setup as required by the protobuf addition.

Contributes to #12

[github-actions]

Package	Line Rate	Health
crates/scion/src	88%	✔
crates/scion/src/address	88%	✔
crates/scion/src/daemon	0%	❌
crates/scion-grpc	0%	❌
crates/scion-grpc/src	0%	❌
Summary	88% (112 / 128)	✔

[jpcsmith]

Looks good.

Question: Is the plan to use the tonic GRPC types and service directly in the socket?

[mlegner]

Question: Is the plan to use the tonic GRPC types and service directly in the socket?

I was actually thinking about this. My plan so far was to use them directly unless a there is a concrete need or use case for some additional abstraction/wrappers.