feat: Allow setting the authentication_mode variable

[mitch-hamm]

Motivation

The aws-auth config map is a legacy feature for cluster authentication. The new approach is outlined and features are described here: https://docs.aws.amazon.com/eks/latest/userguide/access-entries.html#_features
We should allow setting this parameter to have more fine grained control over the migration process from legacy to API control.
The API mode will also give better control to avoid cluster lockouts due to a misconfigured aws-auth config map

As per: https://registry.terraform.io/modules/terraform-aws-modules/eks/aws/20.29.0?tab=inputs

authentication_mode string
Description: The authentication mode for the cluster. Valid values are `CONFIG_MAP`, `API` or `API_AND_CONFIG_MAP`

Default: "API_AND_CONFIG_MAP"

The default is API_AND_CONFIG_MAP

Modifications

Add var for configuration of the authentication_mode and set the default as API_AND_CONFIG_MAP

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• Create a cluster with auth mode set to API_AND_CONFIG_MAP and switch after creation to API
• Create a cluster with auth mode set to API and verify successful creation and functionality

Documentation

Check the box below.

Need to update docs?

• doc-required

  (If you need help on updating docs, create a doc issue)

• no-need-doc

  (Please explain why)

• doc

  (If this PR contains doc changes)

[github-actions]

@mitch-hamm:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)

[github-actions]

@mitch-hamm:Thanks for providing doc info!