Implement EKS support

[ErinCall]

internal/helm.Config will need two new fields:

	EKSCluster string `envconfig:"EKS_CLUSTER"`
	EKSRoleARN string `envconfig:"EKS_ROLE_ARN"`

• internal/run.InitKube and .kubeValues will need matching fields, so their values can be passed along to the kubeconfig template.
• In InitKube.Prepare, if i.EKSCluster != "", i.Token should not be mandatory (and should probably be forbidden).

See also ipedrazas/drone-helm#80 for how this was implemented over there.

[ErinCall]

I'm moving this out of the v1 milestone:

• We don't have an EKS cluster where I could verify the change
• None of the publicly-visible .drone.ymls that use drone-helm are using EKS

I have a branch that's mostly complete, but it can just stay on the backburner until/unless we actually need it.

[Eguthrie3214]

Im currently testing this and have a use case for it. After I confirm that it works maybe you would want to merge it in?

[josmo]

@Eguthrie3214 Definitely and let us know if you need any help :)