Introduce Cargo workspace and update Dockerfiles

Summary:
Introduced cargo workspace for `services`, `shared` and `rust-node-addon`.
Some context [[ https://linear.app/comm/issue/ENG-4630/consider-using-cargo-workspace-for-services#comment-23a2a68a | 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

Author: barthap

.dockerignore

@@ -45,3 +45,5 @@ services/electron-update-server/node_modules
 services/reports/email-config.json
 
 native/cpp/**/build
+
+target

.gitignore

@@ -55,3 +55,6 @@ desktop/out/
 desktop/assets/
 desktop/dist/
 desktop/*.provisionprofile
+
+# Cargo workspace
+target/