Add custom callbacks to ChatInterface buttons

[ahuang11]

Closes #5711

So users can add additional functionality to existing buttons, or add additional buttons easily.

I can't figure out how to drop event here though.

import panel as pn

pn.extension()

pn.chat.ChatInterface(
    custom_callbacks={
        ("Random", "activity"): lambda instance, event: instance.send(
            "Here have a random number"
        ),
        "send": lambda instance, event: instance.send("Hi another send"),
    }
)

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (2867559) 84.16% compared to head (2970c8f) 84.20%.

Files	Patch %	Lines
panel/chat/interface.py	96.36%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5839      +/-   ##
==========================================
+ Coverage   84.16%   84.20%   +0.04%     
==========================================
  Files         291      291              
  Lines       42807    42907     +100     
==========================================
+ Hits        36027    36130     +103     
+ Misses       6780     6777       -3     

Flag	Coverage Δ
ui-tests	40.87% <14.15%> (-0.07%)	⬇️
unitexamples-tests	72.49% <98.23%> (+0.06%)	⬆️

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[ahuang11]

@MarcSkovMadsen in regards to #5711, with this PR, you can do something like:

def reset_welcome(instance, event):
    instance.send(
        state.welcome_message, user="Pandas Agent", avatar="🐼", respond=False
    )
...
ChatInterface(..., button_properties={"reset": {"post_callback": reset_welcome}})

[philippjfr]

Needs rebasing after the merge of the serialize PR also typing_extensions is no longer a dependency so the Self import has to be removed.