Flags to select auth methods for available to `amazonka` by default

[endgame]

By providing appropriate flags, we could toggle the dependency on amazonka-sts and amazonka-sso. This reduces the footprint when compiling things that just have to run on EC2 with a single instance role or whatever.

[endgame]

http://wiki.haskell.org/Cabal/Developer-FAQ#Enabling_additional_features_via_Cabal_flags says:

Question: I like to let the user enable extended functionality using a Cabal flag. Is this the right way?

Answer: Certainly not. Since other packages can distinguish packages only according to their name and their version, it is not a good idea to allow different APIs for the same package version. Cumbersome as it is you have to move extra features to a separate package.

So either we'll need another approach, or will just have to abandon the idea.