gpui: Add support for custom prompt text in PathPromptOptions (#36410)

This will be used to improve the clarity of the git clone UI

### MacOS
<img width="1322" height="128" alt="image"
src="https://github.com/user-attachments/assets/3e511143-12c1-4440-89dd-841b21b2e98e"
/>

### Windows 
<img width="338" height="80" alt="image"
src="https://github.com/user-attachments/assets/766d08d6-0c72-4175-ad24-59dc6188d5f1"
/>

### Linux

<img width="387" height="72" alt="Screenshot From 2025-08-18 15-32-06"
src="https://github.com/user-attachments/assets/3125a7c4-3975-462a-a547-d5d4fac48f22"
/>



Release Notes:

- N/A

crates/gpui/src/shared_string.rs

@@ -23,6 +23,11 @@ impl SharedString {
     pub fn new(str: impl Into<Arc<str>>) -> Self {
         SharedString(ArcCow::Owned(str.into()))
     }
+
+    /// Get a &str from the underlying string.
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
 }
 
 impl JsonSchema for SharedString {