[WIP] fix: ReACT Agent should be consistent with Agent

[MichaelClifford]

Relies on changes in #142 getting merged first.

What does this PR do?

This PR updates the the ReActAgent class so that instantiating it is similar to Agent, with an explicit reliance on the user defining their AgentConfig and passing it as a parameter to the class.

agent = ReActAgent(client=client, agent_config=agent_config)

Closes #161

Test Plan

Tested the changes using a modified version of llama-stack-apps/examples/agents/react_agent.py with the AgentConfig explicitly defined and passed to the ReActAgent like below:

    agent_config = AgentConfig(
        model="meta-llama/Llama-3.1-8B-Instruct",
        enable_session_persistence = False,
        instructions = "You are a helpful assistant.",
        toolgroups = ["builtin::websearch"],
        client_tools = [torchtune],
        tool_choice="auto",
        tool_prompt_format="json",
        )

    agent = ReActAgent(
        client=client,
        agent_config = agent_config
    )

Note: since these changes rely on #142, be aware that this test snippet will likely not work until after the other PR is merged.

[MichaelClifford]

Closing as changes in #142 won't be merged in favor of #178. Can revisit in a followup PR based on #178.