Support setting completions to a different value than parallelism for k8s tasks

[sjawhar]

Requested Functionality

When running a large parallel job on a k8s cluster, I might have many more tasks that need to be run than I have pods to run them. In that case, it would be very useful to use k8s indexed jobs. A key part of this is the ability to set parallelism and completions to different values—specifically, parallelism would have a lower value than completions. See the discussion in https://github.com/iterative/terraform-provider-iterative/pull/597 for background and context.

Example TF file:

resource "iterative_task" "example" {
  cloud     = "k8s"
  machine   = "1-1024"
  image     = "python:3.8.12"
  disk_size = 1

  parallelism = 2
  completions = 5

[0x2b3bfa0]

Related to #585, useful but harder to implement in backends other than k8s

[omesser]

Related to #585, useful but harder to implement in backends other than k8s

@0x2b3bfa0 let's start with k8s support only then

[sjawhar]

You can see here how I implemented it. Should I open a PR, or would you prefer a different implementation?