Add ProxyUrl and uv_toml_only
This adds ProxyUrl type to surface errors earlier to users for invalid
ProxyUrls.
Also adds a uv_toml_only variable that allows us to specify specific
options that are only intended for the uv.toml file and disinclude them
from the generated docs for pyproject.toml examples.
Signed-off-by: Eli Uriegas <eliuriegas@meta.com>
