Expose `existing_kms_instance_crn` as stack variable

[ocofaigh]

Same way it was done here: https://github.com/terraform-ibm-modules/stack-retrieval-augmented-generation/blob/f00047ce548cb8f3570c214aa50bd5c3a5f93cae/stack_definition.json#L57-L63

[Soaib024]

PR merged - hence closing the issue