feat: Add support for OIDC provider

[metral]

Problem description

Create an aws.iam.OpenIdConnectProvider resource for the cluster.

This is necessary to take advantage of the new IAM roles for Pod service account feature: https://aws.amazon.com/about-aws/whats-new/2019/09/amazon-eks-adds-support-to-assign-iam-permissions-to-kubernetes-service-accounts/.

Details:

• https://medium.com/@marcincuber/amazon-eks-with-oidc-provider-iam-roles-for-kubernetes-services-accounts-59015d15cb0c
• https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_providers_create_oidc_verify-thumbprint.html
• https://aws.amazon.com/blogs/opensource/introducing-fine-grained-iam-roles-service-accounts/

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Related: #281