Extract ExtensionIndexedDocsProvider to indexed_docs crate (#20607)

This PR extracts the `ExtensionIndexedDocsProvider` implementation to
the `indexed_docs` crate.

To achieve this, we introduce a new `Extension` trait that provides an
abstracted interface for calling an extension. This trait resides in the
`extension` crate, which has minimal dependencies and can be depended on
by other crates, like `indexed_docs`.

We're then able to implement the `ExtensionIndexedDocsProvider` without
having any knowledge of the Wasm-specific internals of the extension
system.

Release Notes:

- N/A

crates/extension/Cargo.toml

@@ -15,9 +15,11 @@ path = "src/extension.rs"
 anyhow.workspace = true
 async-compression.workspace = true
 async-tar.workspace = true
+async-trait.workspace = true
 collections.workspace = true
 fs.workspace = true
 futures.workspace = true
+gpui.workspace = true
 http_client.workspace = true
 language.workspace = true
 log.workspace = true

crates/extension/src/extension.rs

@@ -1,11 +1,38 @@
 pub mod extension_builder;
 mod extension_manifest;
 
+use std::path::Path;
+use std::sync::Arc;
+
 use anyhow::{anyhow, bail, Context as _, Result};
+use async_trait::async_trait;
+use gpui::Task;
 use semantic_version::SemanticVersion;
 
 pub use crate::extension_manifest::*;
 
+pub trait KeyValueStoreDelegate: Send + Sync + 'static {
+    fn insert(&self, key: String, docs: String) -> Task<Result<()>>;
+}
+
+#[async_trait]
+pub trait Extension: Send + Sync + 'static {
+    /// Returns the [`ExtensionManifest`] for this extension.
+    fn manifest(&self) -> Arc<ExtensionManifest>;
+
+    /// Returns the path to this extension's working directory.
+    fn work_dir(&self) -> Arc<Path>;
+
+    async fn suggest_docs_packages(&self, provider: Arc<str>) -> Result<Vec<String>>;
+
+    async fn index_docs(
+        &self,
+        provider: Arc<str>,
+        package_name: Arc<str>,
+        kv_store: Arc<dyn KeyValueStoreDelegate>,
+    ) -> Result<()>;
+}
+
 pub fn parse_wasm_extension_version(
     extension_id: &str,
     wasm_bytes: &[u8],