feat(connector): [Nexixpay] add mandates flow for cards (#6259)

Co-authored-by: Mrudul Vajpayee <mrudul.vajpayee@mrudulvajpayee-XJWXCWP7HF.local>
Files to modify:


**nexixpay.rs**
  Add:
    - function: private::validate_mandate_payment

**transformers.rs**
  Add:
    - struct: pub::RecurrenceRequest
    - struct: pub::NexixpayNonMandatePaymentRequest
    - struct: pub::NexixpayMandatePaymentRequest
    - struct: pub::BillingAddress
    - struct: pub::PaymentsResponse
    - enum: pub::NexixpayRecurringAction
    - enum: pub::ContractType
    - enum: pub::NexixpayPaymentsRequestData
    - enum: pub::NexixpayPaymentsResponse

**utils.rs**
  Add:
    - function: private::get_optional_shipping_full_name
    - function: private::get_connector_mandate_request_reference_id
    - function: private::get_connector_mandate_request_reference_id

**payment_connector_required_fields.rs**
  Modify existing code