Refactor/Prepare for Multi-language support.

[G8XSU]

Hopefully we add rust support for vss-server in near future.

Planned directory structure:

vss-server/
├── proto/
├── java/
│   ├── build.gradle
│   ├── settings.gradle
│   ├── src/
│   │   ├── main/
│   │   │   ├── java/
│   │   │   └── resources/
│   │   └── test/
│   │       ├── java/
│   │       └── resources/
│   └── (other Java-specific files)
├── rust/
│   ├── Cargo.toml
│   ├── Cargo.lock
│   └── src/
│       ├── main.rs
│       └── (other Rust source files)
├── README.md
├── .gitignore
└── (other repository-level files)

(Please don't merge this yet though.)