Adding support for existing cluster

[dunefro]

1. Add support for existing cluster using variable use_existing_cluster.
2. Following variables are introduced

use_existing_cluster
existing_cluster_node_role_arn
existing_cluster_node_security_group_id
existing_cluster_oidc_issuer_arn
existing_cluster_oidc_issuer_url

3. moved the eks and eks-addons to a count block. This will cause a drift which should be checked in under the migration-guide.md
4. Introduced a scripts folder to ensure better practices for upgrades or any additional use.