[enhancement]: Add encrypted message Subject support

[plkonbklo]

Which feature or improvement would you like to request?

I'd like to see this feature: Some mail clients (Thunderbird/K-9+OpenKeychain/etc) support encrypted message Subject. It'd be great for Stalwart Mail Server to support this as well

Is your feature request related to a problem?

I'm always frustrated when... Sensitive data stored on server is unencrypted.

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