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-state/src/lib.rs

@@ -95,14 +95,17 @@ impl StateStore {
 /// are subdirectories of the state store root.
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
 pub enum StateBucket {
-    // Managed toolchain
+    // Managed toolchains
     Toolchains,
+    // Installed tools
+    Tools,
 }
 
 impl StateBucket {
     fn to_str(self) -> &'static str {
         match self {
             Self::Toolchains => "toolchains",
+            Self::Tools => "tools",
         }
     }
 }