Simple interop between presigned requests and our HTTP client abstractions

[kggilmer]

It is unnecessarily difficult to use PresignedRequest with our own SdkHttpClient interfaces. It requires converting PresignedRequest into HttpRequestBuilder. This isn't terribly difficult but it does feel like a usability concern (it seems completely reasonable that customers may want to re-use the same http engine to fire off a presigned request, especially if they are supplying one to the service client).

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[kggilmer]

I've performed the refactoring such that auth/Presigner.createPresignedRequest() returns type HttpRequestBuilder rather than the presigner-specific type PresignedRequest.

A developer ergonomics issue that comes to mind is that HttpRequestBuilder is a mutable type by design. This is the only type that the native Http Client interface is able to invoke a request from. For internal use cases this makes sense but for usage by customers, it would be more idomatic (and safe) if the client could also use an immutable type such as HttpRequest, preventing unintended mutation of a presigned url before it is executed.

Which leads to a comment in the implementation of SdkHttpClient:

* **NOTE**: This is not a general purpose HTTP client. It is meant for generated SDK use.

This makes me wonder, do we actually want to make SdkHttpClient more usable by customers?

[kggilmer]

Also, it is not straight-forward to adapt a PresignedRequest to a HttpRequestBuilder because information is lost internally in auth/Presigner.createPresignedRequest() when the return type is constructed, specifically the Url components which are required to construct a HttpRequestBuilder.

[kggilmer]

Discussed during standup.. The plan is to remove type PresignedRequest and instead return HttpRequest. The SdkHttpClient and HttpClientEngine will need some work to allow customers to call, but that can occur later. Tracking ticket: #291