Introduce Cargo workspace and update Dockerfiles

Summary:
Introduced cargo workspace for services, shared and rust-node-addon.
Some context in this comment.

I put inline comments to explain it better.

This is the first and most important diff in the stack.

Test Plan:

• Locally: cargo commands inside packages work as before
  • When ran in repo root, e.g. cargo check runs wor the whole workspace
• Docker build times and image sizes are comparable to non-workspace setup
• Pre-commit hooks for Rust still work as before
• CI: All Docker images are built successfully, including Keyserver
• CI: All non-Docker jobs (e.g. Android, iOS) compile Rust successfully
• Ran Services with Commtest locally
• Commtest on CI runs - it requires all services' docker images + its own Docker

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12140