[Turbopack] add telemetry for persistent caching usage (#73554)

### What?

Report persistent caching as feature

crates/next-api/src/project.rs

@@ -823,6 +823,10 @@ impl Project {
             "skipTrailingSlashRedirect",
             *config.skip_trailing_slash_redirect().await?,
         );
+        emit_event(
+            "persistentCaching",
+            *config.persistent_caching_enabled().await?,
+        );
 
         let config = &config.await?;
 

crates/next-core/src/next_config.rs

@@ -413,6 +413,7 @@ pub struct ExperimentalTurboConfig {
     pub tree_shaking: Option<bool>,
     pub module_id_strategy: Option<ModuleIdStrategy>,
     pub minify: Option<bool>,
+    pub unstable_persistent_caching: Option<bool>,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize, TraceRawVcs)]
@@ -1041,6 +1042,17 @@ impl NextConfig {
         Vc::cell(Some(ResolvedVc::cell(rules)))
     }
 
+    #[turbo_tasks::function]
+    pub fn persistent_caching_enabled(&self) -> Result<Vc<bool>> {
+        Ok(Vc::cell(
+            self.experimental
+                .turbo
+                .as_ref()
+                .and_then(|t| t.unstable_persistent_caching)
+                .unwrap_or_default(),
+        ))
+    }
+
     #[turbo_tasks::function]
     pub fn resolve_alias_options(&self) -> Result<Vc<ResolveAliasMap>> {
         let Some(resolve_alias) = self