Rework zkboost to implement proof node API

[han0110]

• Rework this repo to retain only zkboost-{server,client,types} and guest-loader
• Implement the full Proof Node API for zkboost-server with witness fetching (previouly done by EWS)
• Add crate zkboost-client
• Add crate mock-zkattestor to mock zkattestor to test the zkboost-server end to end
• Replace mocknet example with mock-zkattestor
• Add a bash script to download ere guest programs with signature verification
• Update CI to build only zkboost-server image
• Update README.md to replace old API endpoints with new v1 endpoints
• Bump minimum supported Rust version from 1.88 to 1.91