feat: Add Graphviz-based agent visualization functionality

[MartinEBravo]

This pull request introduces functionality for visualizing agent structures using Graphviz. The changes include adding a new dependency, implementing functions to generate and draw graphs, and adding tests for these functions.

New functionality for visualizing agent structures:

• Added graphviz as a new dependency in pyproject.toml.
• Implemented functions in src/agents/visualizations.py to generate and draw graphs for agents using Graphviz. These functions include get_main_graph, get_all_nodes, get_all_edges, and draw_graph.

Testing the new visualization functionality:

• Added tests in tests/test_visualizations.py to verify the correctness of the graph generation and drawing functions. The tests cover get_main_graph, get_all_nodes, get_all_edges, and draw_graph.

For example, given the following code:

from agents import Agent, function_tool
from agents.visualizations import draw_graph


@function_tool
def get_weather(city: str) -> str:
    return f"The weather in {city} is sunny."


spanish_agent = Agent(
    name="Spanish agent",
    instructions="You only speak Spanish.",
)

english_agent = Agent(
    name="English agent",
    instructions="You only speak English",
)

triage_agent = Agent(
    name="Triage agent",
    instructions="Handoff to the appropriate agent based on the language of the request.",
    handoffs=[spanish_agent, english_agent],
    tools=[get_weather],
)


draw_graph(triage_agent)

Generates the following image: