Create type definition for 'http-signature' (#4049)

* Create type definition for 'http-signature'

* Follow lint

src/server/activitypub.ts

@@ -1,7 +1,7 @@
 import { ObjectID } from 'mongodb';
 import * as Router from 'koa-router';
 const json = require('koa-json-body');
-const httpSignature = require('http-signature');
+import * as httpSignature from 'http-signature';
 
 import { createHttpJob } from '../queue';
 import { renderActivity } from '../remote/activitypub/renderer';