✨ Add implementation of fastapi-new CLI, and base for fastapi new command

[savannahostrowski]

This PR adds the initial version of fastapi new, which initializes your project, installs dependencies, and downloads boilerplate code to simplify the fastapi getting started experience.

The goal is to support two core workflows:

# One-off project creation (no install):
uvx fastapi-new my-project

# If they want it integrated:
pip install "fastapi[standard,new]"
fastapi new my-project

The commands above create identical projects with main.py, README.md, and project configuration with a pyproject.toml.

Once this is merged, I will follow up with a PR to fastapi-cli to register this command
and update our docs to reflect this simpler path to getting started. Later, I'd like to extend this to allow users to pass --template or a similar option to customize the boilerplate downloaded.

[patrick91]

Can't wait to have this!

I've left some comments, hopefully I'm not too annoying :D

[tiangolo]

This is looking great! 🤩

I tried it locally to see how it feels and how it looks in the terminal, so polished! I like it. ✨

I just have a few minor comments.

[tiangolo]

I'm realizing we can require Python 3.10+ for running fastapi-new, so we can finally use the new union vertical bar [tears of joy 🥹].

[tiangolo]

Awesome! This looks great! 🚀 ✨ 🎉