Add GitHub Actions Workflow (clippy, fmt, check, test, doc)

[LeWimbes]

Summary

Split out of #69 – this PR adds the new GitHub Actions workflow only.

The workflow runs on every pull request, on every push to main, and it can also be triggered manually. By default it installs the latest (pre-)release of eCAL; when triggered manually, you can specify a particular eCAL version.

At the moment the workflow executes clippy, fmt, check, test, and doc across the entire workspace. Each command runs as an individual job, so they run in parallel and finish in roughly one minute each.

I chose the command‑line flags that looked most reasonable in the docs, but let me know if you’d like any adjustments.

We might want to investigate caching the apt installations. The awalsh128/cache-apt-pkgs-action could cache the regular system packages, but I’m not sure yet how best to handle the eCAL .deb file and its dependencies.

Checklist

• I have tested this change locally
• I have documented any public APIs or CLI changes
• I have added appropriate examples or comments
• The code builds and passes all checks (cargo check, cargo test)
• I have updated the changelog if applicable