Cleanup HttpClient.Transfer

This is just moving fields around. The end result is that there's a
`transfer.req` and a `transfer.res`.

On the Request side, we use to have a nested `params: RequestParam` resulting
in a lot of `transfer.req.params.url`. This is now `transfer.req.url`. On the
Response side, we had the exact opposite: response fields splattered directly
in the transfer, `transfer.response_header`. This is now `transfer.res.header`.

There is now an HttpClient.Response, which is the actual final response (which
could be for a transfer or something else, e.g the cache). And an
HttpClient.Transfer.Response which captures the inflight response data (and is
one of the polymorphic variants of the HttpClient.Response). Probably still not
ideal, but I'm not sure how to make it cleaner, and even if this is just an
intermediary step, I consider it an small win.

src/network/layer/WebBotAuthLayer.zig

@@ -44,8 +44,8 @@ fn request(ptr: *anyopaque, transfer: *Transfer) anyerror!void {
 
     const wba = transfer.client.network.web_bot_auth orelse @panic("WebBotAuthLayer shouldn't be active without WebBotAuth");
 
-    const authority = URL.getHost(transfer.url);
-    try wba.signRequest(transfer.arena, &transfer.req.params.headers, authority);
+    const authority = URL.getHost(transfer.req.url);
+    try wba.signRequest(transfer.arena, &transfer.req.headers, authority);
 
     return self.next.request(transfer);
 }