rename attribute "rate" in Rule object into "rates"

[lecuretFlowbird]

Is your feature request related to a problem? Please describe.

Attribute "rate" is ambiguous because it is used in Rate object to describe a single integer value but object Rule also use it to named an array of object Rate.

Describe the solution you'd like

A better solution could be that attribute in Rule object to be changed to "rates" (in plural like "user_classes")

Is this a breaking change

• Yes, breaking

Impacted Spec

• Curbs

[michael-danko-passport]

I believe @lecuretFlowbird is correct that this can be misleading. While this would be a breaking change, I think we may also want to consider similar clarification outlined within #106