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/indexed_docs/src/store.rs

@@ -324,10 +324,8 @@ impl IndexedDocsDatabase {
             Ok(any)
         })
     }
-}
 
-impl extension_host::DocsDatabase for IndexedDocsDatabase {
-    fn insert(&self, key: String, docs: String) -> Task<Result<()>> {
+    pub fn insert(&self, key: String, docs: String) -> Task<Result<()>> {
         let env = self.env.clone();
         let entries = self.entries;
 
@@ -339,3 +337,9 @@ impl extension_host::DocsDatabase for IndexedDocsDatabase {
         })
     }
 }
+
+impl extension::KeyValueStoreDelegate for IndexedDocsDatabase {
+    fn insert(&self, key: String, docs: String) -> Task<Result<()>> {
+        IndexedDocsDatabase::insert(&self, key, docs)
+    }
+}