feat: add uv version to user agent (#2136)

## Summary

Closes #1977

This allows us to send uv's version in the `uv-client` User Agent
header.

Here's how request headers look like to a server now:
```
...
Accept: application/vnd.pypi.simple.v1+json, application/vnd.pypi.simple.v1+html;q=0.2, text/html;q=0.01
User-Agent: uv/0.1.13
...
```

~~I went for a mix of Option 1 and 2 from #1977.~~ Open to alternative
naming as well, not tied too strongly here to the names picked.

~~Another possibility for this new crate is that we can use it to
consolidate metadata that exists across crates to ultimately be able to
create linehaul information described in #1958, but I haven't looked
into what those changes might look like.~~

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

<!-- How was it tested? -->
Added initial tests in the new crate to exercise its public API and
added a new test to uv-client to validate the headers using a 1-time
disposable server.

crates/uv-version/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "uv-version"
+version = "0.1.14"
+edition = { workspace = true }
+rust-version = { workspace = true }
+homepage = { workspace = true }
+documentation = { workspace = true }
+repository = { workspace = true }
+authors = { workspace = true }
+license = { workspace = true }
+
+[lints]
+workspace = true
+
+[dependencies]

crates/uv-version/src/lib.rs

@@ -0,0 +1,16 @@
+/// Return the application version.
+///
+/// This should be in sync with uv's version based on the Crate version.
+pub fn version() -> &'static str {
+    env!("CARGO_PKG_VERSION")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_get_version() {
+        assert_eq!(version().to_string(), env!("CARGO_PKG_VERSION").to_string());
+    }
+}