lnrpc/invoicesrpc: add new invoice ref LookupInvoiceV2 method

In this commit, we add a new `LookupInvoiceV2` method attached to the
invoice RPC sub-server. Compared to the existing version, this version
allows an invoice to be looked up by an invoice ref. This enables users
to query an AMP invoice based on a specific _set ID_, so they can get
the information related to the set of cumulative settles to that
invoices.

lnrpc/invoicesrpc/invoices.swagger.json

@@ -82,6 +82,52 @@
         ]
       }
     },
+    "/v2/invoices/lookup": {
+      "get": {
+        "summary": "LookupInvoiceV2 attempts to look up at invoice. An invoice can be refrenced\nusing either its payment hash, payment address, or set ID.",
+        "operationId": "Invoices_LookupInvoiceV2",
+        "responses": {
+          "200": {
+            "description": "A successful response.",
+            "schema": {
+              "$ref": "#/definitions/lnrpcInvoice"
+            }
+          },
+          "default": {
+            "description": "An unexpected error response.",
+            "schema": {
+              "$ref": "#/definitions/rpcStatus"
+            }
+          }
+        },
+        "parameters": [
+          {
+            "name": "payment_hash",
+            "in": "query",
+            "required": false,
+            "type": "string",
+            "format": "byte"
+          },
+          {
+            "name": "payment_addr",
+            "in": "query",
+            "required": false,
+            "type": "string",
+            "format": "byte"
+          },
+          {
+            "name": "set_id",
+            "in": "query",
+            "required": false,
+            "type": "string",
+            "format": "byte"
+          }
+        ],
+        "tags": [
+          "Invoices"
+        ]
+      }
+    },
     "/v2/invoices/settle": {
       "post": {
         "summary": "SettleInvoice settles an accepted invoice. If the invoice is already\nsettled, this call will succeed.",