feat: created the OSCR rating feature flag

[nickytonline]

Description

Introduces the oscr-rating feature flag to the codebase. Note that the feature flag has already been created in PostHog.

Related Tickets & Documents

Closes #3641

Mobile & Desktop Screenshots/Recordings

N/A

Steps to QA

N/A

Tier (staff will fill in)

• [ ] Tier 1
• [ ] Tier 2
• [x] Tier 3
• [ ] Tier 4

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