[enhancement]: stalwart-cli: support zstd compressed mails in maildirs

[bjo81]

Which feature or improvement would you like to request?

stalwart-cli can import nested maildirs from dovecot, but not when the messages are zstd compressed

Is your feature request related to a problem?

I exported a mdbox to a nested maildir to import it via stalwart-cli, but the messages are zstd compressed by dovecot. stalwart-cli seems to import the messages, but without content.

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