chore: added support for nat connectivity to the eks worker node subnet for sts calls

[itsharshvb]

This pull request updates the handling of the EKS worker route table to support NAT gateway access in addition to S3, making the infrastructure more flexible for workloads that require outbound internet connectivity. The changes conditionally create and associate the EKS worker route table and its resources based on the enable_nat_gateway variable, and update related outputs and endpoint associations to reflect the new behavior.

EKS Worker Route Table NAT Gateway Support

• The eks_worker_rt module now creates the route table only when enable_nat_gateway is true, and configures it for NAT gateway access instead of S3-only. The route table type tag is updated to "eks-worker-nat" and NAT gateway routes are enabled.
• Route table associations for EKS worker subnets are now conditional on enable_nat_gateway, and reference the route table using indexed access.

Outputs and Endpoint Associations Update

• The output for eks_worker_route_table_id is now conditional on enable_nat_gateway, and returns the route table ID only when NAT gateway is enabled. The description is updated to reflect NAT + S3 support.
• Gateway endpoint route table associations now conditionally include the EKS worker route table only when NAT gateway is enabled, using indexed access.