Add next-secure-headers in example projects

[theodesp]

Tasks

• [ ] I have signed a Contributor License Agreement (CLA) with WP Engine.
• [ ] If a code change, I have written testing instructions that the whole team & outside contributors can understand.
• [ ] I have written and included a comprehensive changeset to properly document the changes I've made.

Description

Adds next-secure-headers in all example projects, providing secure default header configuration across the site.

Related Issue(s):

Testing

-Build and run one of the example projects -Inspect the response headers of each page. It should have the following headers as defined in the package doc

	next-secure-headers	Helmet	Comment
Strict-Transport-Security	forceHTTPSRedirect	hsts
X-Frame-Options	frameGuard	frameguard
X-Download-Options	noopen	ieNoOpen
X-Content-Type-Options	nosniff	noSniff
X-XSS-Protection	xssProtection	xssFilter
Content-Security-Policy	contentSecurityPolicy	contentSecurityPolicy
Expect-CT	expectCT	expectCt
Referrer-Policy	referrerPolicy	referrerPolicy
X-DNS-Prefetch-Control	-	dnsPrefetchControl	This has privacy implications but this improves performance.
Feature-Policy	-	featurePolicy	Feature Policy improves security but it is working draft yet.
X-Powered-By	-	hidePoweredBy	Next.js supports to remove this header in next.config.js.
Related to cache	-	nocache	As Helmet said, caching has lots of benefits.
X-Permitted-Cross-Domain-Policies	-	crossdomain	Adobe Flash is one of old web technologies.

Screenshots

Documentation Changes

Dependant PRs