Add support for inline attachments in SmtpMailer

[johnwc]

Enhanced the Attachment class by adding a ContentId property to support inline content. Updated SmtpMailer to handle attachments with non-empty ContentId as linked resources, generating RFC-compliant ContentId using MimeKit.Utils.MimeUtils.GenerateMessageId(). Modified the HTML body to reference the correct ContentId. Attachments with empty or whitespace ContentId are added as regular attachments.

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• If you are fixing something other than a typo, please always create an issue first. Otherwise, your PR will probably be rejected.
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Based on feature request #376