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/extensions_ui/src/extensions_ui.rs

@@ -1,6 +1,5 @@
 mod components;
 mod extension_context_server;
-mod extension_indexed_docs_provider;
 mod extension_registration_hooks;
 mod extension_slash_command;
 mod extension_suggest;