add eks/fargate distribution with 2-replica StatefulSet

[rmfitzpatrick]

Please do not merge until Splunk distribution has updated k8s_observer and receiver creator components

These changes introduce a new eks/fargate distribution option that adds a receiver creator-controlled kubelet stats receiver to the cluster receiver deployment, as well as a new "cluster receiver observer" deployment to report that collector's kubelet stats metrics since the cluster receiver cannot reach its own kubelet. I chose this dual collector approach over adding the applicable observer/receiver pair to the existing gateway config because there was no clear way to limit discovery to a single gateway* replica and having multiple gateways would lead to redundant metrics (it's also not guaranteed people will use a gateway).

Please see the updated advanced configuration section for a bit more detail.

edit: from @dmitryax's suggestion* I've updated this to be a 2-replica StatefulSet with the same functionality, and pod name based init container logic for node labeling and stripping the cluster receiver on one pod.

[jek-bao-choo]

@dmitryax and @rmfitzpatrick Thank you for this support :)

[rmfitzpatrick]

re: #346 (comment)

I wasn't able to get nested templates to work (unexpected <define> in command) and thought named templates needed to be defined in .tpl, but what I have now seems to allow a multiline string variable* definition.