feat(pyo3): implement flow spec pretty print and add verbose mode

[lemorage]

Following #427 and #404, this PR implements the flow spec pretty print in Rust and adds --verbose mode to enable more detailed output.

Changes on Rust side

1. We first port the code from feat: beautify show cli command with colorful structured output #404, using Rust, and then rewrite it using idiomatic Rust style, largely via Display trait.
2. We add verbose mode to the spec getter method.

Changes on Python side

1. We change it to use get_spec from Rust side.
2. We use rich.Tree instead of rich.Console for clearer readability.

Preview

python main.py cocoindex show DocsToKG

After adding --verbose flag, it will then produce a more detailed output with full operation specs, including all fields and JSON-like formatting, similar to preview in #404.

Resolves #410.

[badmonster0]

Beautiful! Thanks a lot!

[lemorage]

Thanks for all the reviews from @badmonster0. We made a lot of changes since my first comment, so I will try to give a whole summary of them here.

1. We made an enum for distinct output modes in Rust: verbose or concise.
2. We made a trait SpecFormatter to enforce the spec formatting logic per output mode for each opSpec.
3. We made two structs RenderedSpecLine and RenderedSpec, to handle the spec line rendering logic on Rust side, so we can grab those and structure the content via rich.Tree simply on Python side.

So now this is what cocoindex show looks like on the terminal. Basically, for verbose mode, all the nested contents will be unfolded (e.g., a JSON struct), but in concise mode, not.

The following output can be acquired by running python main.py cocoindex show under cocoindex/examples/text_embedding.