[feature] allow for a 3rd party auth handler plugin

[csantanapr]

The OpenWhisk backend allows for SPI implementation to allow different methods of authentication. For example implementing an IAM (Identity Access Manager) for a provider, where the Whisk API can accept Bearer token for example.

I proposed we add an option to the library to take an optional parameter in the constructor to delegate the retrieval of the Authorization Header.
The handler will handle in some cases dealing with an API Key, fetching a token, and refreshing this token.