Add aws_eks_clusters datasource

[Vince-Chenal]

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Relates #13719
Relates #20251

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccAWSEksClustersDataSource_'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -count 1 -parallel 20 -run=TestAccAWSEksClustersDataSource_ -timeout 180m
=== RUN   TestAccAWSEksClustersDataSource_basic
=== PAUSE TestAccAWSEksClustersDataSource_basic
=== RUN   TestAccAWSEksClustersDataSource_empty
=== PAUSE TestAccAWSEksClustersDataSource_empty
=== CONT  TestAccAWSEksClustersDataSource_basic
=== CONT  TestAccAWSEksClustersDataSource_empty
--- PASS: TestAccAWSEksClustersDataSource_empty (22.89s)
--- PASS: TestAccAWSEksClustersDataSource_basic (834.26s)
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws       834.334s

...

As I explained here, I think it's the easiest approach, what do you think ?

[github-actions]

Thank you for your contribution! 🚀

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[ewbankkit]

Similar: #15204.

[ewbankkit]

@Vince-Chenal Thanks for the contribution 🎉 👏.
In order to get this merged quicker I included your commits in #20883.

[github-actions]

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