Setup Protobuf integration module

[Thaleszh]

Is your feature request related to a problem? Please describe.
The SDK currently doesn’t generate or expose types from KiiChain’s Protobufs, which limits compatibility with Cosmos-native tooling and encoding.

Describe the solution you'd like

• Use prost and prost-build to generate Rust structs from KiiChain's .proto files
• Add a proto/ or proto-lib/ crate to the repo
• Export messages like MsgSend, MsgRegisterIdentity, etc.
• Setup a script to recompile Protobufs from the KiiChain repo (possibly via build.rs)

Describe alternatives you've considered
Manual struct definition, but generated protos ensure exact parity with the chain.

Additional context
Use .proto files from kiichain/proto or kiijs-proto and align with buf.build if needed.