Mandatory authorizer field breaks Lambdas without authorizer

[hugoflug]

In 8f0698f, the authorizer field was added to ApiGatewayRequestContext.
However, Lambdas doesn't always have an authorizer. We have a use case where we want to do the auth-checking inside the lambda itself, and because of this change we can't use this library.

Would it be OK to open a PR to make this field a Maybe?

[NickSeagull]

I guess that it wouldn't be any problem, although maybe @dnikolovv could have more context than me for this

[dnikolovv]

I think a Maybe would work great! Sorry for the breakage :(

[NickSeagull]

Closed with #102