Stop leaking isahc assumption (#18408)

Users of our http_client crate knew they were interacting with isahc as
they set its extensions on the request. This change adds our own
equivalents for their APIs in preparation for changing the default http
client.

Release Notes:

- N/A

crates/zed/Cargo.toml

@@ -57,7 +57,6 @@ http_client.workspace = true
 image_viewer.workspace = true
 inline_completion_button.workspace = true
 install_cli.workspace = true
-isahc.workspace = true
 isahc_http_client.workspace = true
 journal.workspace = true
 language.workspace = true