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