Add option to read license key from secret arn instead of passing the secret as a value

[pragmaticivan]

The value is passed as a variable and possibly stored in the terraform state. This is a lambda after all and it can read from the secret manager instead.

Desired Behavior

Have secret_arn as input that can be used instead of license key value.

Possible Solution

Use ARN when available, and fallback to secret value input.