Support remote `https://` requirements files (#1332) (#2081)
## Summary
Allow using http(s) urls for constraints and requirements files handed
to the CLI, by handling paths starting with `http://` or `https://`
differently. This allows commands for such as: `uv pip install -c
https://raw.githubusercontent.com/apache/airflow/constraints-2.8.1/constraints-3.8.txt
requests`.
closes #1332
## Test Plan
Testing install using a `constraints.txt` file hosted on github in the
airflow repository:
https://github.com/jannisko/uv/blob/fbdc2eba8e5e8fb4160baae495daff8fb48df13f/crates/uv/tests/pip_install.rs#L1440-L1484
## Advice Needed
- filesystem/http dispatch is implemented at a relatively low level (at
`crates/uv-fs/src/lib.rs#read_to_string`). Should I change some naming
here so it is obvious that the function is able to dispatch?
- I kept the CLI argument for -c and -r as a PathBuf, even though now it
is technically either a path or a url. We could either keep this as is
for now, or implement a new enum for this case? The enum could then
handle dispatch to files/http.
- Using another abstraction layer like
https://docs.rs/object_store/latest/object_store/ for the
files/urls/[s3] could work as well, though I ran into a bug during
testing which I couldn't debug
