Fix stream ChatMessage for ChatInterface and mention serialize

[ahuang11]

Allows doing this:

    def test_stream_chat_message(self, chat_interface):
        chat_interface.stream(ChatMessage("testeroo", user="useroo", avatar="avataroo"))
        chat_message = chat_interface.objects[0]
        assert chat_message.user == "useroo"
        assert chat_message.avatar == "avataroo"
        assert chat_message.object == "testeroo"

Previously, since it defaulted to self.user it would raise an error.

Also, fixes the typing of the stream; ChatMessage was already supported, just not documented properly.

Lastly, sneaks in a docs mention of the serialize + custom_serializer method in Feed

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 71.59%. Comparing base (67377d7) to head (ec073a6).

Files	Patch %	Lines
panel/chat/interface.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6452      +/-   ##
==========================================
+ Coverage   71.58%   71.59%   +0.01%     
==========================================
  Files         310      310              
  Lines       45955    45975      +20     
==========================================
+ Hits        32896    32916      +20     
  Misses      13059    13059              

Flag	Coverage Δ
unitexamples-tests	71.59% <90.90%> (+0.01%)	⬆️

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