telegram/message: exposed the DropAuthor flag for forwardMessage

[zumoshi]

For this syntax

sender := message.NewSender(api)
sender.To(from).ForwardIDs(to, messageId).Send(ctx)

There was no way to set the DropAuthor flag to true due to the Send method directly creating the MessagesForwardMessagesRequest instance and passing it to Sender.forwardMessages immediately afterwards. Also Sender's methods are private.

This PR tries to enable replicating the Bot API's copyMessage method without having to do the whole process manually replicating existing logic/helpers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.64%. Comparing base (fb1897b) to head (fdac586).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1659      +/-   ##
==========================================
- Coverage   68.90%   68.64%   -0.26%     
==========================================
  Files         446      446              
  Lines       18403    18407       +4     
==========================================
- Hits        12680    12636      -44     
- Misses       4784     4826      +42     
- Partials      939      945       +6     

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