Add given_name and family_name attributes to id token

[alec-w]

Fixes #360

Adds the given_name and family_name attributes into the id token.

In our project making use of cognito-local (thanks for this) this addressed the problem for us.

Guidance would be helpful on whether this is indeed the correct fix and how the tests should be modified to prevent a future regression on these attributes (typescript not being my forte).

[jagregory]

I think this looks fine. Ideally, we'd have a canonical list of attributes which are supposed to be exposed via the tokens and then we could write tests which verify they're exposed (if they're set on the user), but unless someone volunteers to do that I'm happy to just merge.

[cirnatdan]

@jagregory is it possible to merge this now? Btw Cognito actually exposes given_name and family_name only if they are set as required attributes

[hugo-abhishek]

@jagregory Are we planning to merge this?
@alec-w Will it be possible for you to include preferred_username as well? We have use this attribute in one of our projects and this would be really helpful.