Remove AWS4AuthLayer and HTTP.request extension

[ararslan]

It's currently exceedingly difficult for packages to extend HTTP.request by providing a custom layer, which is likely why the logic for an AWS authentication layer is in HTTP rather than AWSCore. I found this out while attempting and failing to write a test that properly exercises the request extension here.

The simplest course of action would be to remove the functionality from this package and instead introduce a dependency on AWSAuth to HTTP, with the request extension using signing defined here.

[codecov-io]

Codecov Report

Merging #10 into master will increase coverage by 5.47%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
+ Coverage   76.66%   82.14%   +5.47%     
==========================================
  Files           2        2              
  Lines          60       56       -4     
==========================================
  Hits           46       46              
+ Misses         14       10       -4

Impacted Files	Coverage Δ
src/AWSAuth.jl	100% <ø> (ø)	⬆️
src/signaturev4.jl	81.81% <ø> (+5.54%)	⬆️

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[rofinn]

Maybe make an issue in HTTP.jl describing why it's difficult to extend HTTP.request with custom layers? This seems like an issue that should be addressed in any further design/refactoring and should be documented.

[iamed2]

To be explicit, the hard part is adding the layer to the stack right? The actual creation of the custom layer is easy but getting HTTP to use it is hard?

[ararslan]

Yes, what Eric said is exactly right. One can define a Layer subtype with a request method that does what you want, but there are some serious complications. One example is that not every request(::Layer, args...) method takes the same arguments, so to know how to formulate the next call to request in your method, you need to know where in the stack your layer is, so that you know the expected signature for the next call. Internally, HTTP gets around this by having HTTP.stack define the layers in a specific order that works. In that case, the AWS authentication layer is not the outermost layer, and to ensure that this package puts the layer in the same spot, it would need to completely reconstruct the stack by hand, and ensure that any preprocessing that's done in the user-facing request methods that get called before those which take Layers is taken care of. That essentially amounts to a reimplementation of HTTP.request.

[ararslan]

I've opened an issue on HTTP for discussion: JuliaWeb/HTTP.jl#389