Allow adding multiple blinded routes for a blinded path config

[alexbosworth]

Follow up to the feature request to inform invoice creation with specific inbound peers, a limitation highlighted by @bitromortac:

With #9127 it is now possible to specify a list of incoming channel ids in the BlindedPathConfig in AddInvoice, which constrains the blinded route construction to that chain of hops, so this covers some requirements from this issue. One current caveat here is that one can only add a single route instead of multiple ones and adding a BuildBlindedRoute could be more flexible.

Originally posted by @bitromortac in #9673

So the followup needed would be that you should be able to add multiple routes rather than just a single one