add limited s/mime compatibility when sending email - PoC

[tomholub]

#1158 addresses a similar issue in even more limited ways - some enterprises have a way of converting s/mime certs to openpgp public keys, when are missing self-signatures

The above is easier, but it would be better to instead have a way to support s/mime certs natively

[tomholub]

part of enterprise requests #2125

[tomholub]

Could do a PoC in four steps (possibly four PRs sequentially?):

1. PoC in a separate folder, like flowcrypt-browser/smime-poc. Does not have to use TypeScript. Allows me to put a public certificate in one textarea, message in another textarea, click a button and shows encrypted result in a pre.
2. Move the PoC to become htm/ts files in flowcrypt-browser/extension/chrome/dev, with libraries in flowcrypt-browser/extension/lib, with code style closer to the rest of the repo, but still otherwise separate from all workflows.
3. Generalize and start integrating with code, possibly add to compose window but only on Firefox, marked as experimental (that's what we usually do - fewer users on Firefox, so we only allow experimental features there). Add tests.
4. Let customers try it and give us feedback, then adjust as needed (compatibilty?), then unmark as experimental and allow on Chrome.