feat: added dry-run mode for content_metadata transmission

[zamanafzal]

Ticket: https://2u-internal.atlassian.net/browse/ENT-5959

Adds support for dry run mode to work with transmission of content metadata.

Dry Run mode could be enabled using this flag.

Merge checklist:

• Any new requirements are in the right place (do not manually modify the requirements/*.txt files)
  • base.in if needed in production but edx-platform doesn't install it
  • test-master.in if edx-platform pins it, with a matching version
  • make upgrade && make requirements have been run to regenerate requirements
• make static has been run to update webpack bundling if any static content was updated
• ./manage.py makemigrations has been run
  • Checkout the Database Migration Confluence page for helpful tips on creating migrations.
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.
  • This should be run from either a venv with all the lms/edx-enterprise requirements installed or if you checked out edx-enterprise into the src directory used by lms, you can run this command through an lms shell.
    • It would be ./manage.py lms makemigrations in the shell.
• Version bumped
• Changelog record added
• Translations updated (see docs/internationalization.rst but also this isn't blocking for merge atm)

Post merge:

• Tag pushed and a new version released
  • Note: Assets will be added automatically. You just need to provide a tag (should match your version number) and title and description.
• After versioned build finishes in GitHub Actions, verify version has been pushed to PyPI
  • Each step in the release build has a condition flag that checks if the rest of the steps are done and if so will deploy to PyPi.
    (so basically once your build finishes, after maybe a minute you should see the new version in PyPi automatically (on refresh))
• PR created in edx-platform to upgrade dependencies (including edx-enterprise)
  • This must be done after the version is visible in PyPi as make upgrade in edx-platform will look for the latest version in PyPi.
  • Note: the edx-enterprise constraint in edx-platform must also be bumped to the latest version in PyPi.

[johnnagro]

depending on how you intent to consume/parse these dry-run logs you may want to make the logging more granular

[zamanafzal]

Thank you @johnnagro for your valuable feedback.

[alex-sheehan-edx]

A side effect of this implementation is that content transmission audits will retain the marked_for field value set by the exporter (as it's the job of the transmitter to remove the value). Theoretically this is fine as the value simply denotes the exporter identifying the content needs updating in some way, but we should double check that we want the dry run to effect records in the database (perhaps it's even a benefit?).

[zamanafzal]

@johnnagro @alex-sheehan-edx I've added course_run_key variable in the generate_log_line method.