DT-982: Refactor support ticket models

[rushtong]

Addresses

Partially addresses https://broadworkbench.atlassian.net/browse/DT-982

Summary

This PR refactors the model classes used for submitting Zendesk tickets. Future PRs will introduce more functionality such as adding API endpoints and real file uploads.

• Whenever possible, classes are converted to records.
• This adds uploads to the comments model. Actual file uploads is not yet supported, but this allows for them to be specified in the request itself.
• While we're here, add the API docs for the yaml. SupportTicket.yaml corresponds to what is currently sent via the UI as an entity. SupportTicketResponse.yaml is the exact format Zendesk returns after a POST operation.

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Have you read CONTRIBUTING.md lately? If not, do that first.

• Label PR with a Jira ticket number and include a link to the ticket
• Label PR with a security risk modifier [no, low, medium, high]
• PR describes scope of changes
• Get a minimum of one thumbs worth of review, preferably two if enough team members are available
• Get PO sign-off for all non-trivial UI or workflow changes
• Verify all tests go green
• Test this change deployed correctly and works on dev environment after deployment

[pshapiro4broad]

Looks OK, just a few comments

[rjohanek]

refactor looks good!