addPaymentToOrder is not channel aware

[martijnvdbrug]

Describe the bug
Payments are channel aware, but addPaymentToOrder mutation does not adhere to the channel awareness: It gets the first PaymentMethod by code and enabled=true

To Reproduce
Steps to reproduce the behavior:

1. Create a payment-method in default channel
2. Make sure you don't have payments in channel2
3. addPaymentToOrder on channel2
4. The order now has a payment from defaultchannel

Expected behavior
An error to be thrown, because no method is found

Environment (please complete the following information):

• @vendure/core version: 1.0.0-beta-4
• Nodejs version v12
• Database (mysql/postgres etc): mysql

Additional context
https://github.com/vendure-ecommerce/vendure/blob/master/packages/core/src/service/services/payment-method.service.ts#L137

[martijnvdbrug]

Currently working on this issue: running the test before creating a PR now