Add support for specifying worker version

[robertbartel]

Allow jobs to be requested using a version of the worker other than the default (i.e., the latest tag).

Closes #662.

Changes

• Added attribute to ngen and ngen-cal job requests for worker_version, that defaults to latest if not explicitly set
• Modified determine_image_for_job function in Launcher class to use the new worker_version attribute in nested job request object of ngen and ngen-cal jobs, rather than always just latest
• Updated CLI parsing for job exec to accept a --worker-version
• Modified versions and dependency relationships as needed for involved services
• Modified unit tests to match new deserialization structure

Testing

1. Automated tests pass

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Visually tested in supported browsers and devices (see checklist below 👇)
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

[aaraney]

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