multi: add blinded paths to invoices

Expose the ability to add blinded paths to an invoice. Also expose
various configuration values.

We also let the lncfg.Invoices struct satisfy the Validator interface so
that we can verify all its config values in one place.

sample-lnd.conf

@@ -1649,6 +1649,40 @@
 ; enough to prevent force closes.
 ; invoices.holdexpirydelta=12
 
+; The minimum number of real (non-dummy) blinded hops to select for a blinded
+; path. This doesn't include our node, so if the maximum is 1, then the
+; shortest paths will contain our node along with an introduction node hop.
+; invoices.blinding.min-num-real-hops=1
+
+; The number of hops to include in a blinded path. This does not include
+; our node, so if is is 1, then the path will at least contain our node along
+; with an introduction node hop. If it is 0, then it will use this node as
+; the introduction node. This number must be greater than or equal to the
+; the number of real hops (invoices.blinding.min-num-real-hops). Any paths
+; shorter than this number will be padded with dummy hops.
+; invoices.blinding.num-hops=2
+
+; The maximum number of blinded paths to select and add to an invoice.
+; invoices.blinding.max-num-paths=3
+
+; The amount by which to increase certain policy values of hops on a blinded
+; path in order to add a probing buffer. The higher this multiplier, the more
+; buffer is added to the policy values of hops along a blinded path meaning
+; that if they were to increase their policy values before the blinded path
+; expires, the better the chances that the path would still be valid meaning
+; that the path is less prone to probing attacks. However, if the multiplier
+; is too high, the resulting buffered fees might be too much for the payer.
+; invoices.blinding.policy-increase-multiplier=1.1
+
+; The amount by which to decrease certain policy values of hops on a blinded
+; path in order to add a probing buffer. The lower this multiplier, the more
+; buffer is added to the policy values of hops along a blinded path meaning
+; that if they were to increase their policy values before the blinded path
+; expires, the better the chances that the path would still be valid meaning
+; that the path is less prone to probing attacks. However, since this value
+; is being applied to the MaxHTLC value of the route, the lower it is, the
+; lower payment amount will need to be.
+; invoices.blinding.policy-decrease-multiplier=0.9
 
 [routing]