Allow extensions to define providers for indexing docs (#13755)

This PR provides extensions with the ability to define providers for
indexing docs.

Release Notes:

- N/A

crates/extension/src/extension_manifest.rs

@@ -76,6 +76,8 @@ pub struct ExtensionManifest {
     pub language_servers: BTreeMap<LanguageServerName, LanguageServerManifestEntry>,
     #[serde(default)]
     pub slash_commands: BTreeMap<Arc<str>, SlashCommandManifestEntry>,
+    #[serde(default)]
+    pub indexed_docs_providers: BTreeMap<Arc<str>, IndexedDocsProviderEntry>,
 }
 
 #[derive(Clone, Default, PartialEq, Eq, Debug, Deserialize, Serialize)]
@@ -137,6 +139,9 @@ pub struct SlashCommandManifestEntry {
     pub requires_argument: bool,
 }
 
+#[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
+pub struct IndexedDocsProviderEntry {}
+
 impl ExtensionManifest {
     pub async fn load(fs: Arc<dyn Fs>, extension_dir: &Path) -> Result<Self> {
         let extension_name = extension_dir
@@ -200,5 +205,6 @@ fn manifest_from_old_manifest(
             .collect(),
         language_servers: Default::default(),
         slash_commands: BTreeMap::default(),
+        indexed_docs_providers: BTreeMap::default(),
     }
 }