Change method signatures to add the context parameter

[ifraixedes]

In order to make the package more usable from the point of view of better concurrency control the methods should implement the standard Go pattern of passing the Context as a first parameters of the functions.

I make this proposal because AWS SDK implements the same functions but with the context parameter as a first parameter

[ifraixedes]

Making this change involve to do change the signature of the 20 methods that the AWSReader defines and the only one current implementation that the repository has.

If AWS SDK has a replicated function that we use with the context as a parameter and there isn't appear some replicated functions which more than the context parameter as required, than the ones without the context parameters, the refactoring shouldn't take more than 1 working day.

Doing it right now, will make the refactoring shorter than in the future, in case that new AWS services be added