zed_extension_api: Add HttpRequestBuilder (#16165)

This PR adds an `HttpRequestBuilder` to the extension API to allow for a
more ergonomic way for constructing HTTP requests within extensions.

The HTTP client functionality is now also exposed via the
`zed_extension_api::http_client` module instead of top-level.

Release Notes:

- N/A

crates/extension_api/src/settings.rs

@@ -1,3 +1,5 @@
+//! Provides access to Zed settings.
+
 #[path = "../wit/since_v0.1.0/settings.rs"]
 mod types;