This commit makes HTTP client parameters used in `fetch` API
configurable on the extension initialization via a callback
`client_builder_hook` that users can provide.
The main motivation behind this change is to allow `deno_fetch` users to
tune the HTTP/2 client to suit their needs, although Deno CLI users will
not benefit from it as no JavaScript interface is exposed to set these
parameters currently.
It is up to users whether to provide a hook function. If not provided,
the default configuration from hyper crate will be used.
Ref #26785
Sending ALPN to a proxy, and then when tunneling, requires better
juggling of TLS configs. This improves the choice of TLS config in the
proxy connector, based on what reqwest does. It also includes some
`ext/fetch/tests.rs` that check the different combinations.
Fixes #24632
Fixes #24691
