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_host/src/wasm_host/wit.rs

@@ -3,12 +3,11 @@ mod since_v0_0_4;
 mod since_v0_0_6;
 mod since_v0_1_0;
 mod since_v0_2_0;
+use extension::KeyValueStoreDelegate;
 use lsp::LanguageServerName;
 use release_channel::ReleaseChannel;
 use since_v0_2_0 as latest;
 
-use crate::DocsDatabase;
-
 use super::{wasm_engine, WasmState};
 use anyhow::{anyhow, Context, Result};
 use language::LspAdapterDelegate;
@@ -422,15 +421,15 @@ impl Extension {
         store: &mut Store<WasmState>,
         provider: &str,
         package_name: &str,
-        database: Resource<Arc<dyn DocsDatabase>>,
+        kv_store: Resource<Arc<dyn KeyValueStoreDelegate>>,
     ) -> Result<Result<(), String>> {
         match self {
             Extension::V020(ext) => {
-                ext.call_index_docs(store, provider, package_name, database)
+                ext.call_index_docs(store, provider, package_name, kv_store)
                     .await
             }
             Extension::V010(ext) => {
-                ext.call_index_docs(store, provider, package_name, database)
+                ext.call_index_docs(store, provider, package_name, kv_store)
                     .await
             }
             Extension::V001(_) | Extension::V004(_) | Extension::V006(_) => {