* ✨ Enable db_subnet_group_name variable

If the variable not specify then use the db_subnet_group module

* Disable create_db_subnet_group if db_subnet_group_name is defined

* Support conditional creation for the database too.

* Database instance outputs need to accommodate the case where count is zero.

* Declare variables that allow conditional creation of the database, parameter group, and subnet group. Pass them from main.tf to the nested modules.

* Consistently accommodate boolean like arguments.

aws_db_parameter_group (or AWS?) does not support removing previously
defined parameters.

I thought I'd give "taint" a spin and force the parameter group to be
recreated.  terraform tried to delete the parameter group but failed as
it's still in use.

This change allows terraform to migrate the RDS instance to the new
parameter group before destroying the old.

Accessing aws_db_subnet_group.this when its count = 0 is an error, so we
need to instead access it via splat syntax and then apply a default value
for when the resulting list is empty.

Added iam_database_authentication_enabled and license_model model

Added complete example for Postgres RDS

support final_snapshot_identifier

Update README

Add enhanced monitoring

Using `storage_encrypted` to set the db_instance.storage_encrypted