Add configureOptions closure to `perform` to allow editing configuration options in-line with perform

[nakulbajaj]

Brought up here for PR #60

I think in many cases, there may be just a few options that developers will want to set for their perform calls that a way to configure options in-line would be nice. I'm currently working on some code that's adopting the API, and all I really need is something that sets URLSession's assumesHTTP3Capable, and nothing else.

We showed this to Maciej who liked having it as well since it avoided the need to separately initialize and options and then set the fields, as well as being able to use the $0 convenience within the closure instead of typing out the variable name every time.

[FranzBusch]

Instead of adding this every method could we potentially add this to RequestOptions themselves? Maybe something like

extension RequestOptions {
  static func build<RequestOptions>(body: (inout Self) -> Void) -> Void {}
}

I haven't tried the above but it would be great if it doesn't require another parameter on every single method.