feat: Update GraphQL schema to create or update charges

[vincent-pochet]

Context

We want to introduce a new charge model to charge users based on a consumed volume.

The existing graduated pricing does not fit this need because it creates tier, and each tier has its own price.
The volume price works differently, it creates tiers, but the whole number of units is multiplied by the current tier. Then, the number of units consumed determine the total price for each unit.

Description

This PR updates the GraphQL schema to allow creation, update and query of the Volume charge model.

• A new VolumeRange type is added to handle range definition. Attributes are fromValue, toValue, perUnitAmount and flatAmount
• The Charge type is updated to add a new volumeRanges attribute as an array of VolumeRange
• A new VolumeRangeInput type is added to handle range creation and update. Arguments are fromValue, toValue, perUnitAmount and flatAmount
• The ChargeInput type used in createPlan and updatePlan is also updated to handle a new argument volumeRangeInput as an array of VolumeRangeInput

Related Task

This PR follows #394 and #404