feat: Add Segment events for CheckoutIntent state transitions

[zwidekalanga]

Track state changes in CheckoutIntent lifecycle by emitting appropriate Segment analytics events on transitions

Description:
Add a description of your changes here.

Jira:
ENT-XXXX

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