feat: add capacity type node groups to allow for spot nodes (#152)

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### Motivation

This PR will allow us to specify the capacity type, which is typically
"on-demand". There are some instances where we would like to make node
groups spot for cost savings on lower level environments.

### Modifications

I've exposed the `node_pool_capacity_type` which allows modification to
the capacity types of all node groups created via the
[eks-managed-node-group](https://github.com/terraform-aws-modules/terraform-aws-eks/tree/50cb230c8a1793f5ef4cc52c4c789b656b141168/modules/eks-managed-node-group#eks-managed-node-group-module)
submodule.

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---------

Signed-off-by: Max Xu <[email protected]>
Co-authored-by: Max Xu <[email protected]>

Author: dpappa