feat(frontier): add rpc to get billing account portal url

raystack/frontier/v1beta1/frontier.proto

@@ -1460,6 +1460,15 @@ service FrontierService {
     };
   }
 
+  rpc GetBillingAccountPortalURL(GetBillingAccountPortalURLRequest) returns (GetBillingAccountPortalURLResponse) {
+    option (google.api.http) = {get: "/v1beta1/organizations/{org_id}/billing/{id}/portal"};
+    option (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_operation) = {
+      tags: "Billing";
+      summary: "Get billing account portal URL";
+      description: "Get a billing account portal url by ID.";
+    };
+  }
+
   rpc GetBillingBalance(GetBillingBalanceRequest) returns (GetBillingBalanceResponse) {
     option (google.api.http) = {get: "/v1beta1/organizations/{org_id}/billing/{id}/balance"};
     option (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_operation) = {
@@ -1832,6 +1841,19 @@ message DeleteBillingAccountRequest {
 
 message DeleteBillingAccountResponse {}
 
+message GetBillingAccountPortalURLRequest {
+  // ID of the billing account to get
+  string id = 1 [(validate.rules).string.min_len = 1];
+
+  string org_id = 2 [(validate.rules).string.min_len = 1];
+
+  string return_url = 3;
+}
+
+message GetBillingAccountPortalURLResponse {
+  string url = 1;
+}
+
 message GetBillingBalanceRequest {
   // ID of the billing account to get the balance for
   string id = 1 [(validate.rules).string.uuid = true];