Email Routing subdomain support, new APIs and security protocols

This post introduces support for Authenticated Received Chain (ARC) and MTA Strict Transport Security (MTA-STS) for Cloudflare Email Routing. ARC preserves email authentication results (SPF, DKIM, DMARC) through intermediaries by adding `Arc-Authentication-Results`, `Arc-Message-Signature`, and `Arc-Seal` headers. MTA-STS is enabled by creating a CNAME record for `_mta-sts` and deploying a Worker to serve a policy file from `mta-sts.example.com/.well-known/mta-sts.txt`. Additionally, new Email Workers APIs allow sending emails from scratch and programmatically processing incoming emails, enhancing the capabilities of the Email Routing service.