feat: upcoming invoice amount due

[iloveagent57]

http://localhost:18270/api/v1/stripe-event-summary/first_upcoming_invoice_amount_due/?subscription_plan_uuid=aaaabbbb-db33-455d-80ca-0153dd1e94fa

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[pwnage101]

Nice! Aside from the comments above, LGTM.

[pwnage101]

TIL create_preview() is a replacement for the deprecated upcoming().

[brobro10000]

1 comment about code cleanup to avoid duplicative code.

[brobro10000]

I believe this class also exists within the following directory.
enterprise_access/apps/customer_billing/tests/test_stripe_event_handlers.py

Might be worth doing a bit of cleanup and generalizing as a common test helper function to avoid duplicative code.

[brobro10000]

Approved to unblock. I'd wait for either @iloveagent57 or @pwnage101 for a final review before merging.