alpstable#400 update decode pipeline

[prestonvasquez]

Resolves #400

This update removed the "Decode" function from the exported API in favor of a functional pattern for decoding based on the application/content-type. The new method for decoding an HTTP response body was given a minor optimization:

Before:

goos: darwin
goarch: arm64
pkg: github.com/alpstable/gidari
BenchmarkDecodeUpsertRequest-10             1203            927933 ns/op          314197 B/op      10059 allocs/op

After:

goos: darwin
goarch: arm64
pkg: github.com/alpstable/gidari
BenchmarkDecodeUpsertRequest-10             5685            219681 ns/op          378087 B/op      10070 allocs/op