Feature request: `InitiateAuth` should return `ExpiresIn` according to the value set on the corresponding User Pool Client

[RobHoman]

Catching whatever discrepancies I can and feeding them your way!

Noticed that responses for InitiateAuth are not setting the ExpiresIn value.

https://docs.aws.amazon.com/cognito-user-identity-pools/latest/APIReference/API_InitiateAuth.html#CognitoUserPools-InitiateAuth-response-AuthenticationResult

https://docs.aws.amazon.com/cognito-user-identity-pools/latest/APIReference/API_AuthenticationResultType.html#CognitoUserPools-Type-AuthenticationResultType-ExpiresIn