Add uv tool install (#4492)

This is the minimal "working" implementation. In summary, we:

- Resolve the requested requirements
- Create an environment at `$UV_STATE_DIR/tools/$name`
- Inspect the `dist-info` for the main requirement to determine its
entry points scripts
- Link the entry points from a user-executable directory
(`$XDG_BIN_HOME`) to the environment bin
- Create an entry at `$UV_STATE_DIR/tools/tools.toml` tracking the
user's request

The idea with `tools.toml` is that it allows us to perform upgrades and
syncs, retaining the original user request (similar to declarations in a
`pyproject.toml`). I imagine using a similar schema in the
`pyproject.toml` in the future if/when we add project-levle tools. I'm
also considering exposing `tools.toml` in the standard uv configuration
directory instead of the state directory, but it seems nice to tuck it
away for now while we iterate on it. Installing a tool won't perform a
sync of other tool environments, we'll probably have an explicit `uv
tool sync` command for that?

I've split out todos into follow-up pull requests:

- #4509 (failing on Windows)
- #4501 
- #4504 

Closes #4485

crates/uv-cli/src/lib.rs

@@ -1838,6 +1838,8 @@ pub struct ToolNamespace {
 pub enum ToolCommand {
     /// Run a tool
     Run(ToolRunArgs),
+    /// Install a tool
+    Install(ToolInstallArgs),
 }
 
 #[derive(Args)]
@@ -1881,6 +1883,46 @@ pub struct ToolRunArgs {
     pub python: Option<String>,
 }
 
+#[derive(Args)]
+#[allow(clippy::struct_excessive_bools)]
+pub struct ToolInstallArgs {
+    /// The command to install.
+    pub name: String,
+
+    /// Use the given package to provide the command.
+    ///
+    /// By default, the package name is assumed to match the command name.
+    #[arg(long)]
+    pub from: Option<String>,
+
+    /// Include the following extra requirements.
+    #[arg(long)]
+    pub with: Vec<String>,
+
+    #[command(flatten)]
+    pub installer: ResolverInstallerArgs,
+
+    #[command(flatten)]
+    pub build: BuildArgs,
+
+    #[command(flatten)]
+    pub refresh: RefreshArgs,
+
+    /// The Python interpreter to use to build the tool environment.
+    ///
+    /// By default, uv will search for a Python executable in the `PATH`. uv ignores virtual
+    /// environments while looking for interpreter for tools. The `--python` option allows
+    /// you to specify a different interpreter.
+    ///
+    /// Supported formats:
+    /// - `3.10` looks for an installed Python 3.10 using `py --list-paths` on Windows, or
+    ///   `python3.10` on Linux and macOS.
+    /// - `python3.10` or `python.exe` looks for a binary with the given name in `PATH`.
+    /// - `/home/ferris/.local/bin/python3.10` uses the exact Python at the given path.
+    #[arg(long, short, env = "UV_PYTHON", verbatim_doc_comment)]
+    pub python: Option<String>,
+}
+
 #[derive(Args)]
 #[allow(clippy::struct_excessive_bools)]
 pub struct ToolchainNamespace {