Added 'perMethodTaxRate' field to CustomLineItems

To align with the HTTP API, we now added support for the perMethodTaxRate field on CustomLineItems to the GraphQL API.

Changes:

• [GraphQL API] Added the perMethodTaxRate field to the CustomLineItem type.

The following changes were introduced in terms of GraphQL SDL:

extend type CustomLineItem {
  perMethodTaxRate: [MethodTaxRate!]!
}