Introduce [pwsh] keyword

[AdhocAdam]

If a digital signature can be cryptographically verified, then a Sender could invoke:

• pre-written PowerShell
• a Rest API
• a webhook
• execute PowerShell directly from email (e.g. call SCOM, SCCM, various Runbooks, Test-Connection, etc.)

While this feature's use would be entirely optional and disabled by default. The use of MimeKit and email signatures would be mandatory to ensure environment security.

[AdhocAdam]

This will be released via #226