Add Amazon Alexa Skills Kit event

[kylevanek]

Alexa Skills Kit event type with response builder

[fsenart]

Hi @kylevanek,

First of all thank you very much for your contribution.
A few remarks about the PR:

• Can you please migrate nested anonymous structs into standalone named structs for the sake of visibility and maintainability.
• Can you please remove the response builder part from this PR. Indeed, this project purpose is to only provide the core event structures and the idea is to provide more useful/value added libs on top of it. As an example, the API Gateway proxy event is represented here only with its structure and used in eawsy/aws-lambda-go-net with more value added.
• We have not yet a contribution guideline (our bad), but by contributing to the project you agree to transfer the copyright of the code changes to Alsanium.