Add Handling of 'License' message to the annotation proto.

This information is used by the default html doc generator and so it is nice
to be able to provide it.

Add the message to openapiv2.proto and extend template.go to handle it.

Tested via:
bazel build //examples/proto/examplepb:expamplepb_protoc_gen_swagger
more bazel-bin/examples/proto/examplepb/a_bit_of_everything.swagger.json

examples/proto/examplepb/a_bit_of_everything.proto

@@ -19,6 +19,10 @@ option (grpc.gateway.protoc_gen_swagger.options.openapiv2_swagger) = {
       url: "https://github.com/grpc-ecosystem/grpc-gateway";
       email: "none@example.com";
     };
+    license: {
+      name: "BSD 3-Clause License";
+      url: "https://github.com/grpc-ecosystem/grpc-gateway/blob/master/LICENSE.txt";
+    };
   };
   // Overwriting host entry breaks tests, so this is not done here.
   external_docs: {

protoc-gen-swagger/genswagger/template.go

@@ -784,6 +784,17 @@ func applyTemplate(p param) (string, error) {
 					s.Info.Contact.Email = spb.Info.Contact.Email
 				}
 			}
+			if spb.Info.License != nil {
+				if s.Info.License == nil {
+					s.Info.License = &swaggerLicenseObject{}
+				}
+				if spb.Info.License.Name != "" {
+					s.Info.License.Name = spb.Info.License.Name
+				}
+				if spb.Info.License.Url != "" {
+					s.Info.License.URL = spb.Info.License.Url
+				}
+			}
 		}
 		if spb.Host != "" {
 			s.Host = spb.Host

protoc-gen-swagger/options/openapiv2.proto

@@ -74,8 +74,7 @@ message Info {
   string description = 2;
   string terms_of_service = 3;
   Contact contact = 4;
-  // field 5 is reserved for 'license'.
-  reserved 5;
+  License license=5;
   string version = 6;
 }
 
@@ -90,6 +89,17 @@ message Contact {
   string email = 3;
 }
 
+// `License` is a representation of OpenAPI v2 specification's License object.
+//
+// See: https://github.com/OAI/OpenAPI-Specification/blob/3.0.0/versions/2.0.md#licenseObject
+//
+message License {
+  // Required. The license name used for the API.
+  string name = 1;
+  // A URL to the license used for the API.
+  string url = 2;
+}
+
 // `ExternalDocumentation` is a representation of OpenAPI v2 specification's
 // ExternalDocumentation object.
 //