feat(minimum-commitment): Add minimum commitment

[ivannovosad]

Roadmap Task

👉 https://getlago.canny.io/feature-requests/p/set-minimum-spend-for-subscriptions

Context

As a user, I want to define the minimum spend that applies to the customer's plan. Consider the following monthly plan:

• Subscription fee = $50 per month (in arrears);
• Usage-based charge = $10 per unit per month (in arrears); and
• Monthly minimum spend = $100.

If at the end of the month, the customer has consumed 3 units, their total invoice will be: $50 + 3 * $10 = $80 + $20 (true-up fee) = $100.

Description

This PR adds minimum commitment to plan and plan overrides.