[BACKEND] Set up endpoint for admin to make date(s) available for booking

[bcho892]

Is your feature request related to a problem? Please describe.

The request body should contain:

{
Date: Timestamp[],
Description: optional,
Title: string
}

We want to create a new product (if required) in stripe with the details using #125 and also create a firestore document in the booking_slots using #186 (if required). We should NOT allow duplicate stripe products to be created so this should be checked first.

[!NOTE] You may want to first check firestore for a document with the date, if it exists, change the max_slots status to > 0

Also remove the stripe_product_id from booking_slot, replace with is_active

BEFORE MERGING

• [ ] Integration test written for services
• [ ] Schemas annotated if adding new models
• [ ] Code generation run (hint: yarn workspace server tsoa spec-and-routes)
• [ ] Appropriate mocks created where possible
• [ ] PR Reviewed (For non-trivial changes)
• [ ] Changes tested after rebasing on master or merging in master (hint: git fetch origin master:master, then git rebase master or git merge master)
• [ ] All required PR checks passing

[bcho892]

dont require removing stripe_product_id anymore because that's done in #133