Adds capability to attach existing security groups to Opensearch domain

[pierreislande]

what

• Adds capability to attach existing security groups to Opensearch domain by setting var.create_security_group to false
• Almost all the necessary settings already exist to support this functionality, as it aligns with how the module currently handles security group configuration for the OpenSearch domain
• This feature already exists for Elasticsearch domain

why

• This modification enables users to either create a new security group or specify existing ones via the var.security_groups variable. This is especially useful for integrating with externally managed security groups.
• We expect to use existing security groups to attach to the OpenSearch domain in cases where we don’t want to use the security group created by the module.
• The var.security_groups variable already exists and accepts a list of security group IDs that can be directly applied to the OpenSearch domain.

references

https://github.com/cloudposse/terraform-aws-elasticsearch/pull/134