feat(ext/fetch): support fallible request-builder hooks

[lucab]

This tweaks the signature of request_builder_hook in order to support fallible hooks.

The rationale for this is mostly on two sides:

• it allows a hook to inspect and possibly drop an outgoing request (e.g. for policying purposes), bubbling up a detailed error message to the user.
• it wires into newer reqwest API which allows to split and then reassemble a RequestBuilder, although only in a fallible way (async/request: add methods to split and reassemble a RequestBuilder seanmonstar/reqwest#1770)

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[ry]

LGTM

[satyarohith]

LGTM

[satyarohith]

The description of the PR says it uses the new RequestBuilder::from_parts() but I don't see it in the code. I am probably misunderstanding the intent of the second point?

[lucab]

@satyarohith RequestBuilder::from_parts() is not yet in the latest reqwest released version, so it doesn't directly appear here. But the idea is that the consumer-provided hook logic can now split and re-assemble the RequestBuilder to manipulate the inner Request, something like this:

fn custom_hook(input: RequestBuilder) -> Result<RequestBuilder, AnyError> {
  let (client, inner_req) = input.build_split();
  let req = inner_req?;
  // ... Manipulate `req` as needed here ...
  let output = RequestBuilder::from_parts(client, req);
  Ok(output)
}

[satyarohith]

Understood. Thanks for the explanation!