feat: basics of business logic for customer provisioning

[iloveagent57]

Implements business logic to create new customers with pending admins. The response schema looks like this:

{
    "enterprise_customer": {
        "uuid": "e4912161-55fe-435f-9a9c-c510f241910e",
        "name": "Test customer B",
        "country": "US",
        "slug": "test-customer-b"
    },
    "customer_admins": {
        "created_admins": [],
        "existing_admins": [
            {
                "user_email": "[email protected]"
            },
            {
                "user_email": "[email protected]"
            },
            {
                "user_email": "[email protected]"
            }
        ]
    }
}

Jira:
ENT-10071

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

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