Add cognito utilities

[Nxtra]

I'm thinking of adding some Cognito utilities:

• checkUserExists
• checkUserStatusEquals
  ...
  Would you like to have this added?

[joelhamiltondev]

Hi! These sounds great. There's now an assertion merged in which checks that a user pool contains a user with a specific username, and there's a PR up for refactoring it to also check that a user has the correct attributes :)

The user status assertion sounds great! Feel free to submit a PR and I'll check it out! :)