Introduce OpenAPI specs (#4351)

* wip

* wip

* wip

* Update index.ts

* Update gen-openapi-spec.ts

* Update api.ja-JP.md

* Fix

* Improve doc

* Update gen-openapi-spec.ts

* Update redoc.html

* Improve doc

* Update gen-openapi-spec.ts

* Improve doc

* Update CHANGELOG.md

src/server/api/endpoints.ts

@@ -2,18 +2,30 @@ import { Context } from 'cafy';
 import * as path from 'path';
 import * as glob from 'glob';
 
+export type Param = {
+	validator: Context<any>;
+	transform?: any;
+	default?: any;
+	desc?: { [key: string]: string };
+	ref?: string;
+};
+
 export interface IEndpointMeta {
 	stability?: string; //'deprecated' | 'experimental' | 'stable';
 
 	desc?: { [key: string]: string };
 
+	tags?: string[];
+
 	params?: {
+		[key: string]: Param;
+	};
+
+	errors?: {
 		[key: string]: {
-			validator: Context<any>;
-			transform?: any;
-			default?: any;
-			desc?: { [key: string]: string };
-			ref?: string;
+			message: string;
+			code: string;
+			id: string;
 		};
 	};