Add --extra to uv add and enable fine grained updates (#4566)

## Summary

- Adds a `--extra` flag to `uv add` that allows activating extras
without the PEP508 syntax.
- `uv add` now errors if the update is ambiguous (e.g. the dependency is
present twice with different markers)
- `uv add` is smarter about updates. For example, `uv add flask==3.0.0`
followed by `uv add flask --extra dotenv` preserves the previous version
specifier.

Resolves https://github.com/astral-sh/uv/issues/4419.

crates/uv-cli/src/lib.rs

@@ -1771,6 +1771,10 @@ pub struct AddArgs {
     #[arg(long)]
     pub branch: Option<String>,
 
+    /// Extras to activate for the dependency; may be provided more than once.
+    #[arg(long)]
+    pub extra: Option<Vec<ExtraName>>,
+
     #[command(flatten)]
     pub installer: ResolverInstallerArgs,