lnrpc/routerrpc+routing: add new MaxShardAmt field to LightningPayment

lnrpc/routerrpc/router_backend.go

@@ -565,6 +565,14 @@ func (r *RouterBackend) extractIntentFromSendRequest(
 	}
 	payIntent.MaxParts = maxParts
 
+	// If this payment had a max shard amount specified, then we'll apply
+	// that now, which'll force us to always make payment splits smaller
+	// than this.
+	if rpcPayReq.MaxShardSizeMsat > 0 {
+		shardAmtMsat := lnwire.MilliSatoshi(rpcPayReq.MaxShardSizeMsat)
+		payIntent.MaxShardAmt = &shardAmtMsat
+	}
+
 	// Take fee limit from request.
 	payIntent.FeeLimit, err = lnrpc.UnmarshallAmt(
 		rpcPayReq.FeeLimitSat, rpcPayReq.FeeLimitMsat,

routing/router.go

@@ -1684,6 +1684,14 @@ type LightningPayment struct {
 	// MaxParts is the maximum number of partial payments that may be used
 	// to complete the full amount.
 	MaxParts uint32
+
+	// MaxShardAmt is the largest shard that we'll attempt to split using.
+	// If this field is set, and we need to split, rather than attempting
+	// half of the original payment amount, we'll use this value if half
+	// the payment amount is greater than it.
+	//
+	// NOTE: This field is _optional_.
+	MaxShardAmt *lnwire.MilliSatoshi
 }
 
 // SendPayment attempts to send a payment as described within the passed