Make it easy to use single agent

[sternakt]

Why are these changes needed?

This PR will add the functionality to run ConversibleAgent and its subclasses as standalone agents which can write and execute code and independently call functions and tools

x_assistant = AssistantAgent(name="x_assistant", llm_config=llm_config)

browser_use_tool = BrowserUseTool(
    llm_config=llm_config,
)

hot_topic_res = x_assistant.run(
    "Find out today's hot topic and an influencer who is talking about it on X using a web search",
    tools=browser_use_tool,
)

print(hot_topic_res)

Related issue number

Closes #523

Checks

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• I've added tests (if relevant) corresponding to the changes introduced in this PR.
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