Added backend JWKS endpoint caching

refs https://github.com/TryGhost/Toolbox/issues/411
refs f58b5984cb

- Backend JWKS endpoint did not have any HTTP caching
- It is recommended to cache JWKS endpoints (ref.: https://docs.apigee.com/api-platform/reference/policies/jwt-policies-overview)
- This change also brings the endpoint up to speed with what members JWKS
endpoint does
- Above point gives another reminder to extract JWKS functionality into a
common module that should be reused

ghost/core/core/server/web/well-known.js

@@ -1,5 +1,7 @@
+const cacheControl = require('@tryghost/mw-cache-control');
 const express = require('../../shared/express');
 const settings = require('../../shared/settings-cache');
+const config = require('../../shared/config');
 
 module.exports = function setupWellKnownApp() {
     const wellKnownApp = express('well-known');
@@ -14,7 +16,9 @@ module.exports = function setupWellKnownApp() {
         return keyStore.toJSON();
     };
 
-    wellKnownApp.get('/jwks.json', async (req, res) => {
+    const cache = cacheControl('public', {maxAge: config.get('caching:wellKnown:maxAge')});
+
+    wellKnownApp.get('/jwks.json', cache, async (req, res) => {
         const jwks = await getSafePublicJWKS();
 
         // there's only one key in the store atm

ghost/core/test/e2e-server/__snapshots__/well-known.test.js.snap

@@ -16,6 +16,7 @@ Object {
 
 exports[`.well-known GET /jwks.json should return a JWKS 2: [headers] 1`] = `
 Object {
+  "cache-control": "public, max-age=86400",
   "content-length": "277",
   "content-type": "application/json; charset=utf-8",
   "etag": StringMatching /\\(\\?:W\\\\/\\)\\?"\\(\\?:\\[ !#-\\\\x7E\\\\x80-\\\\xFF\\]\\*\\|\\\\r\\\\n\\[\\\\t \\]\\|\\\\\\\\\\.\\)\\*"/,