feat: index enrollStart and hasEnrollStart in Algolia course_runs

[adamstankiewicz]

• Includes enroll_start_date in the NormalizedContentMetadataSerializer. This will persist this field in the json_metadata blob for course records.
• Includes enroll_start in the indexed course runs metadata for course objects within Algolia; this will primarily be consumed by Admin Portal during the assignment allocation flow.

Post-review

• Squash commits into discrete sets of changes
• Ensure that once the changes have been deployed to stage, prod is manually deployed

[adamstankiewicz]

[inform] Updates the course run type to be Exec Ed when dealing with the late enrollment cases, as late enrollment is typically used with Exec Ed content.

[brobro10000]

Smart idea to serialize this value as part of normalized metadata. Approving to unblock, but I'd also have someone else take a look at this as well, just as a sanity check 👍🏽