General Enhancements in agentchat 2.0 (#1906)

* work in progress

* wip

* groupchat type hints

* clean up

* formatting

* formatting

* clean up

* address comments

* better comment

* updates docstring a_send

* resolve comments

* agent.py back to original format

* resolve more comments

* rename carryover type exception

* revert next_agent changes + keeping UndefinedNextagent

* fixed ciruclar dependencies?

* fix cache tests

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Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>
Co-authored-by: Chi Wang <wang.chi@microsoft.com>