feat(multi-connection): create payment provider customer in dedicated mutation (#6054)

## Context

Currently customers can be attached to only one connection of the same
type. In order to change it, Lago is adding possibility to have multiple
connections of the same type per customer.

## Description

This PR adds mutation to create payment provider customer. Currently it
is possible to do it along with customer creation. This is the dedicated
mutation, created only for this purpose.