[feature]: create new `BuildBlindedRoute` RPC for the router sub-server

[Roasbeef]

Is your feature request related to a problem? Please describe.

Today it isn't possbile to give lnd a series of pubkeys, then have it create a blinded path blob, assuming that the first node is the introduction point for the route.

Describe the solution you'd like

Add a new BuildBlindedRoute RPC call which implements the above. The logic should automatically look up the various routing policies for each of the nodes to populate the routing policy information for a given routing.BlindedPayment.

A caller should then be able to take the response of the new call, and pass it into an AddInvoice call, which does additional verification, then utilizes the blinded hop information as is.

Describe alternatives you've considered

We could extend the existing BuildRoute for this purpose, but the return values are rather different.

[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.