per-request timeout override

[jaytaph]

All timeouts are currently global in FetcherConfig and apply to every request equally.
There is no way to give a video stream total_body_timeout: None while keeping a 30-second
limit on CSS.

[jaytaph]

Besides the total_body_timeout, there are other options in the FethcerConfig that we might want to set per request.

By adding this to the FetchRequest struct, it means that setting up this struct becomes more boilerplate, so we might want to get some kind of functionality that creates it.

Maybe with a builder pattern:

let fr = FetchRequest::default()
    .withTotalBodyTimeout(Duration::seconds(30))
    .withUserAgent(CUSTOM_USER_AGENT_STRING)
    .asStream()
    .....
    .build();;