added support to create additional user pool clients to manage multiple apps

[NikitaGandhi]

Why this change is needed

Describe why this change is needed, what issues it will fix and the benefits the change will add

• We wanted use same cognito user pool for multiple applications, e.g. jupyterhub, airflow. This allows us to manage users who can have access to multiple applications using same user account.

Negative effects of this change

Will making this change break or change an existing functionality? flag it here

• No impact. Introduce new variable app_clients to configure user pool app clients. check the README usage.

[NikitaGandhi]

solve #208

[NikitaGandhi]

@woodcockr As part of this PR we wanted to deprecate some variables and resource. Please review it and make necessary changes so we can safely remove them.