Add silent option in nested chats and group chat

[robraux]

Why are these changes needed?

The silent parameter is useful when dealing with deeply orchestrated workflows on the CLI. This adds additional support for respecting this useful configuration in the more complex orchestrators.

• Adds silent option to GroupChatManager to mirror other silent configurations in Agents
• Respects silent request in Nested Chats to removed otherwise to avoidable __post_carryover_processing messaging.

Decisions

• Deliberately leaves introductions and broadcast replies as silent=True as these add no value to display.
• GroupChat silent configuration tweaked to support the plugin use case without adding additional vars to ConversableAgent.

Related issue number

Replaces PR: #2212
Closes: #1335

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation 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.

[robraux]

@robraux please read the following Contributor License Agreement(CLA). If you agree with the CLA, please reply with the following information.

@microsoft-github-policy-service agree [company="{your company}"]

Options:

• (default - no company specified) I have sole ownership of intellectual property rights to my Submissions and I am not making Submissions in the course of work for my employer.

@microsoft-github-policy-service agree

• (when company given) I am making Submissions in the course of work for my employer (or my employer has intellectual property rights in my Submissions by contract or applicable law). I have permission from my employer to make Submissions and enter into this Agreement on behalf of my employer. By signing below, the defined term “You” includes me and my employer.

@microsoft-github-policy-service agree company="Microsoft"

Contributor License Agreement

@microsoft-github-policy-service agree