feat: Add support for HTML emails in outgoing messages (#12662)

This PR adds sending custom HTML content in outgoing email messages
through Chatwoot's Email channels, while maintaining backward
compatibility with existing markdown rendering.

###  API Usage

**Endpoint:** `POST
/api/v1/accounts/{account_id}/conversations/{conversation_id}/messages`

```json
  {
   "content": "Fallback text content",
   "email_html_content": "<div><h1>Welcome!</h1><p>This is <strong>custom HTML</strong></p></div>"
  }
```

---------

Co-authored-by: Muhsin <muhsinkeramam@gmail.com>

app/views/mailers/conversation_reply_mailer/email_reply.html.erb

@@ -1,9 +1,11 @@
-<% if @message.content %>
-  <%= ChatwootMarkdownRenderer.new(@message.outgoing_content).render_message %>
+<% if @message.content_attributes.dig('email', 'html_content', 'reply').present? %>
+<%= @message.content_attributes.dig('email', 'html_content', 'reply').html_safe %>
+<% elsif @message.content %>
+<%= ChatwootMarkdownRenderer.new(@message.outgoing_content).render_message %>
 <% end %>
 <% if @large_attachments.present? %>
-  <p>Attachments:</p>
-  <% @large_attachments.each do |attachment| %>
-    <p><a href="<%= attachment.file_url %>" target="_blank"><%= attachment.file.filename.to_s %></a></p>
-  <% end %>
+<p>Attachments:</p>
+<% @large_attachments.each do |attachment| %>
+<p><a href="<%= attachment.file_url %>" target="_blank"><%= attachment.file.filename.to_s %></a></p>
+<% end %>
 <% end %>