Add Token Streaming in AGS , Support Env variables

[victordibia]

Adds Token Streaming in AGS.

Agentchat now supports streaming of tokens via ModelClientStreamingChunkEvent . This PR is to track progress on supporting that in the AutoGen Studio UI.

If model_client_stream is enabled in an assitant agent, then token will be streamed in UI.

streaming_assistant = AssistantAgent(
    name="assistant",
    model_client=model_client,
    system_message="You are a helpful assistant.",
    model_client_stream=True,  # Enable streaming tokens.
)

ags_streaming_720.mov

Also adds support for env variables in AGS Settings

You can set env variables that are loaded just before a team is run. Handy to set variable to be used by tools etc.

Note: the set variables are available to the server process.

Why are these changes needed?

Related issue number

Closes #5627

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.56%. Comparing base (a226966) to head (c50d0aa).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5659   +/-   ##
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  Coverage   75.56%   75.56%           
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  Files         171      171           
  Lines       10483    10483           
=======================================
  Hits         7921     7921           
  Misses       2562     2562           

Flag	Coverage Δ
unittests	75.56% <ø> (ø)

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