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/copilot/Cargo.toml

@@ -37,7 +37,6 @@ fs.workspace = true
 futures.workspace = true
 gpui.workspace = true
 http_client.workspace = true
-isahc.workspace = true
 language.workspace = true
 lsp.workspace = true
 menu.workspace = true