Add checks to sieve validator (mime and foreverypart)

[melone2]

Allowing the inclusion of the sieve extensions mime and foreverypart would allow for additional sieve filter options.
The usecase could be filtering mails by attached filetypes.
e.g. redirecting or blocking mails which have a certain file attached to them.

With the proposed changes I was able to successfully save a sieve filter script with following content:

require ["mime", "foreverypart", "copy"];

foreverypart
{
  if header :mime :anychild :param "filename" :matches "Content-Disposition" ["*.pdf"]
  {
    redirect :copy "copycat@example.com";
  }
}

This script will forward every mail that has a .pdf file attached to a separate E-Mail address (e.g. dms such as paperless-ngx).

Removing the "filename" string from this example caused the parser to fail - as it should.

[milkmaker]

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