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/platform.rs

@@ -1278,7 +1278,7 @@ pub enum WindowBackgroundAppearance {
 }
 
 /// The options that can be configured for a file dialog prompt
-#[derive(Copy, Clone, Debug)]
+#[derive(Clone, Debug)]
 pub struct PathPromptOptions {
     /// Should the prompt allow files to be selected?
     pub files: bool,
@@ -1286,6 +1286,8 @@ pub struct PathPromptOptions {
     pub directories: bool,
     /// Should the prompt allow multiple files to be selected?
     pub multiple: bool,
+    /// The prompt to show to a user when selecting a path
+    pub prompt: Option<SharedString>,
 }
 
 /// What kind of prompt styling to show