Update workers to respect default command from configuration class

[desertaxle]

This PR make a couple of small updates:

• Updates the default command for both the Cloud Run worker and the Vertex AI worker to come from the configuration class instead of being hardcoded
• Add pytest-xdist to make tests run faster in CI

Example

Screenshots

Checklist

• References any related issue by including "Closes #" or "Closes ".
  • If no issue exists and your change is not a small fix, please create an issue first.
• Includes tests or only affects documentation.
• Passes pre-commit checks.
  • Run pre-commit install && pre-commit run --all locally for formatting and linting.
• Includes screenshots of documentation updates.
  • Run mkdocs serve view documentation locally.
• Summarizes PR's changes in CHANGELOG.md