MTA-STS and TLS-RPT for Proton Mail Custom Domains

# MTA-STS & TLS-RPT Setup for Proton Mail Custom Domainss
Proton Mail doesn't provide instructions for setting up MTA-STS and TLS-RPT, probably because it's likely to make migrations to other services difficult.

After setting it up you can verify MTA-STS and TLS-RPT with https://easydmarc.com/tools/mta-sts-check and https://easydmarc.com/tools/tls-rpt-check. You can also check your Cloudflare logs where you'll see daily lookups by e.g. Gmail.

## Setting up MTA-STS
Doing it Cloudflare style:
1. Create a Worker for the subdomain `mta-sts`.
2. Use the following script:
```
export default {
  async fetch(request, env, ctx) {
    return fetch('https://mta-sts.protonmail.ch/.well-known/mta-sts.txt');
  },
};
```
3. Create a CNAME with the name `_mta-sts` pointing to `_mta-sts.protonmail.ch`.

## Setting up TLS-RPT
Doing it the same way Proton does for their domains, e.g. `pm.me`.
It's unclear what Proton will do with the reports, if anything.

1. Create a TXT record with the name `_smtp._tls`
2. Enter the value `v=TLSRPTv1; rua=https://reports.proton.me/reports/smtptls`.