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/indexed_docs.rs

@@ -1,7 +1,9 @@
+mod extension_indexed_docs_provider;
 mod providers;
 mod registry;
 mod store;
 
+pub use crate::extension_indexed_docs_provider::ExtensionIndexedDocsProvider;
 pub use crate::providers::rustdoc::*;
 pub use crate::registry::*;
 pub use crate::store::*;