Cognito module: support token validity units

[NikitaGandhi]

Description Enahace Cognito module to support token validity units.

AWS Cognito allows configuring the token validity units of tokens for each User Pool Client [1]. This configuration are still missing from terraform documentation [2].

Reference: [1] https://docs.aws.amazon.com/cli/latest/reference/cognito-idp/create-user-pool-client.html [2] https://github.com/terraform-providers/terraform-provider-aws/issues/14919

Affected Resource(s) aws_cognito_user_pool_client

Potential Module Configuration

  token_validity_units {
     access_token  = "hours"
     id_token      = "hours"
     refresh_token = "days"
     // the above are defaults; valid values are: seconds | minutes | hours | days
  }