[DAG] Add DAG Visualization with Jupyter Support

[andylizf]

Resolve #4277.

Adds a plot method to the Dag class to visualize task dependencies. Automatically displays inline in Jupyter or saves to a file in other environments.

Example:

from sky import Dag, Task

with Dag() as dag:
    task_a = Task(name="A")
    task_b = Task(name="B")
    task_c = Task(name="C")
    task_a >> task_b >> task_c

dag.plot()

Notes:

• Requires matplotlib and networkx.

Tested (run the relevant ones):

• [x] Code formatting: bash format.sh
• [x] Any manual or new tests for this PR (please specify below)
  • Tested in Jupyter Notebook (.ipynb)
  • Tested in regular Python environment
• [ ] All smoke tests: pytest tests/test_smoke.py
• [ ] Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• [ ] Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh

[andylizf]

@cblmemo PTAL, thanks!

[andylizf]

Hi @cblmemo, could you handle the merge for me?