Add tiers to plan resource

[florisvdg]

This PR adds tiers to the plan resource.

resource "stripe_plan" "some_plan" {
  ...

  tier {
    up_to       = 20
    unit_amount = 0
  }

  tier {
    up_to       = 100
    unit_amount = 350
  }

  tier {
    up_to_inf   = true
    unit_amount = 250
  }
}

It's a ForceNew field, because Stripe's plan update endpoint doesn't support it.

[franckverrot]

Sounds great! I'd love to put an example in main.tf for people to have an example at hand. Would you like to do one more commit or have me do it? I'll merge when it's in. Thanks!

[florisvdg]

Done. Could you also create a release for it?

[franckverrot]

Here you go https://github.com/franckverrot/terraform-provider-stripe/releases/tag/v1.1.0

Thanks so much again, please tell me if everything's alright and sorry for the delay.