feat: allow staff users to override lms_user_id in can-redeem API

[adamstankiewicz]

Description:

Supports an optional lms_user_id query parameter in the can-redeem API to override the default user id used for the request. Only works if the authenticated request user is_staff=True.

http://localhost:18270/api/v1/policy-redemption/enterprise-customer/{enterprise_customer_uuid}/can-redeem/?lms_user_id=1234

Jira:
ENT-9291

Merge checklist:

• ./manage.py makemigrations has been run
  • 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.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance

[iloveagent57]

Looks good, few minor nits.