Add the ability to send an email with a LinkedResource

[Stanislaw-Rudnicki]

This pull request adds support for linked resources in the EmailMessage model. It also updates the ToMimeMessage method to handle these linked resources, ensuring they are properly included in email messages. A new constructor is introduced to maintain backward compatibility.

Changes:

1. Updated the EmailMessage class:

   • Added a new property LinkedResources of type IList<EmailMessageLinkedResource>.
   • Introduced a new constructor to avoid breaking changes.
   • Implemented HasLinkedResources for checking linked resource presence.

2. Added a new EmailMessageLinkedResource class:

   • Supports streams for linked resources in emails.

3. Updated the ToMimeMessage extension method:

   • Enhanced it to include LinkedResources when converting to MimeMessage.

---

Testing Steps:

1. Unit Test for Backward Compatibility:

   • Create an EmailMessage object using the old constructor (without LinkedResources).
   • Verify that the LinkedResources property is null.
   • Ensure the ToMimeMessage method works as expected, without errors.

2. Unit Test for New Functionality:

   • Create an EmailMessage object using the new constructor, including LinkedResources:
   
   

   using var stream = new FileStream(
           Path.Combine(_env.WebRootPath, "images", "logo.png"),
           FileMode.Open,
           FileAccess.Read
       );
   
   var contentId = MimeUtils.GenerateMessageId();
   var linkedResource = new EmailMessageLinkedResource(stream, "logo.png", contentId);
       
   var email = new EmailMessage(
           from: "[email protected]",
           to: new[] { "[email protected]" },
           cc: null,
           bcc: null,
           replyTo: null,
           subject: "Test Email",
           body: $"<img src=\"cid:{contentId}\" />",
           isBodyHtml: true,
           attachments: null,
           linkedResources: new[] { linkedResource }
       );

   • Verify that the ToMimeMessage method includes the linked resource with the correct ContentId.

3. Edge Cases:

   • Test with empty or null values for LinkedResources to ensure no exceptions occur.
   • Pass a mix of valid and null entries in the LinkedResources collection and ensure only valid resources are included.

4. Integration Testing:

   • Use the updated EmailMessage model in an actual email-sending flow.
   • Verify that emails with linked resources render correctly in email clients (e.g., embedded images display as expected).

5. Code Review:

   • Verify that changes maintain compatibility with existing functionality and do not introduce breaking changes.

---

Expected Results:

• Emails sent without linked resources should behave as before.
• Emails with linked resources should correctly embed those resources with specified ContentId.
• No exceptions or regressions should occur during testing.

[github-actions]

Hi there @Stanislaw-Rudnicki, thank you for this contribution! 👍

While we wait for one of the Core Collaborators team to have a look at your work, we wanted to let you know about that we have a checklist for some of the things we will consider during review:

• It's clear what problem this is solving, there's a connected issue or a description of what the changes do and how to test them
• The automated tests all pass (see "Checks" tab on this PR)
• The level of security for this contribution is the same or improved
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• Avoids creating breaking changes; note that behavioral changes might also be perceived as breaking
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[lauraneto]

Hi @Stanislaw-Rudnicki ,

Thank you for the pull request! 🙌
Your code changes were introducing a breaking change, so I left a comment on how to make it non breaking.

Would you also be able to update the description of the pull request to include testing steps? That would be very helpful!
Thanks! 🙏