Support quote/outgoing payment to STREAM connection

[wilsonianb]

A quote's specified receiver may be either:

The URL of the incoming payment or ILP STREAM connection that is being paid.

The backend's quote creation currently returns an error if a resolved payment destination is not an incoming payment:

rafiki/packages/backend/src/open_payments/quote/service.ts

Lines 197 to 204 in 668e7cb

	if (!destination.destinationPaymentDetails) {
	deps.logger.warn(
	{
	options
	},
	'missing incoming payment'
	)
	throw new Error('missing incoming payment')

[wilsonianb]

• feat(backend): add Open Payments client service #595 removes calls to Pay.setupPayment by query incoming payment urls directly. It is then able to construct a ResolvedPayment to pass to either Pay.startQuote or Pay.pay.

However, after querying an Open Payments connection when attempting to create a fixed-send quote, we are unable to populate the ResolvedPayment's destinationAsset because we do not yet know the receiver's asset details. Pay.setupPayment accomplishes this by performing an asset probe.

Possible workarounds include:

1. include the receiver's asset details in the /connections/{connectionId} response (the asset of the corresponding incoming payment).
2. call Pay.setupPayment in Rafiki's Open Payments client service with the connection's destinationAddress and sharedSecret for the sole purpose of performing the asset probe.