feat: Add custom completer for cargo +<TAB> to complete toolchain name (#15301)

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### What does this PR try to resolve?

Related to #14520

This PR introduces auto-completion for the `cargo +<TAB>` option. When a
user types `cargo +<TAB>` and presses the TAB key, the system will
automatically suggest toolchain in current device

### How should we test and review this PR?

To verify this feature, follow these steps:
1. In the terminal, type `cargo +<TAB>`
2. Press the TAB key.
3. You should see toolchain suggestions

https://github.com/user-attachments/assets/c8bd8a5e-e97d-4d0a-b284-1ed52734c4a2

Author: epage

src/bin/cargo/cli.rs

@@ -691,9 +691,32 @@ See '<cyan,bold>cargo help</> <cyan><<command>></>' for more information on a sp
                 }))
             }).collect()
         })))
+        .add(clap_complete::engine::SubcommandCandidates::new(|| {
+            get_toolchains_from_rustup()
+                .into_iter()
+                .map(|t| clap_complete::CompletionCandidate::new(t))
+                .collect()
+        }))
         .subcommands(commands::builtin())
 }
 
+fn get_toolchains_from_rustup() -> Vec<String> {
+    let output = std::process::Command::new("rustup")
+        .arg("toolchain")
+        .arg("list")
+        .arg("-q")
+        .output()
+        .unwrap();
+
+    if !output.status.success() {
+        return vec![];
+    }
+
+    let stdout = String::from_utf8(output.stdout).unwrap();
+
+    stdout.lines().map(|line| format!("+{}", line)).collect()
+}
+
 #[test]
 fn verify_cli() {
     let gctx = GlobalContext::default().unwrap();