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
0e2f6e38e7