Test "Hello World" gRPC server and client

The Rust gRPC server listens on localhost port 50051.

1. To successfully build the Rust server, the protoc compiler must be installed separately. Make sure it is on the current path, or the PROTOC environment variable is set to the path of the binary. It can be downloaded from:

https://github.com/protocolbuffers/protobuf/releases

2. To build and run the Rust server, run:

cargo run --bin server

3. To build and run the Java gRPC client, run:

mvn install exec:java

Experimental gRPC interface for Bisq2 Musig2 trade protocol

There is a (highly) experimental gRPC interface being developed for the Musig2 trade protocol, currently bundled in the same helloworld.proto file as the above. (TODO: Organise and move to a more appropriate place.) A Java client conducting a dummy two-party trade can be invoked by running:

mvn exec:java -Pmusig

The Rust code uses the musig2 crate to construct aggregated signatures for the traders' warning and redirect transactions, with pubkey & nonce shares and partial signatures exchanged with the Java client, to pass them back in as fields of the simulated peer's RPC requests, setting up the trade.

The adaptor logic, multiparty signing and simulated steps for the whole of the trade (both normal and force-closure via the swap tx) are now implemented for the mockup, but none of the mediation, arbitration or claim paths are implemented or mocked yet. Dummy messages to represent the txs to sign are currently being used in place of real txs built with the aid of BDK or a similar wallet dependency.

See MuSig trade protocol messages for my current (incomplete) picture of what the trade messages between the peers would look like, and thus the necessary data to exchange in an RPC interface between the Bisq2 client and the Rust server managing the wallet and key material.