Drop Kuber.jl dependency

[omus]

Using Kuber.jl seems like more trouble than it's worth at the moment.

• Kuber objects only support very basic getfield and setfield access requiring a work around when non-valid field strings are required. e.g. the updated GPU configuration function mentioned in the README would need to be written as:

  function my_gpu_configurator(pod)
      limits = something(pod.spec.containers[1].resources.limits, Dict())
      limits["nvidia.com/gpu"] = 1
      pod.spec.containers[1].resources.limits = limits
      return pod
  end

• kube_obj requires a JSON string or Dict. It cannot accept AbstractDict.
• Kuber.jl is the reason we're required to start the kubectl proxy command
• KuberContext has been problematic in the CI environment (https://github.com/beacon-biosignals/K8sClusterManagers.jl/pull/31#issuecomment-824087436)
• We want to move towards having users being able to pass in the worker pod manifest

Update: Additional reasons to get rid of Kuber.jl:

• Sending too many requests through the kubectl proxy can cause exceptions to be raised which require requests to be retried. Such failures can also cause the kubectl proxy to crash which results in the user needing to know how to restart the proxy command

[codecov-commenter]

Codecov Report

Merging #34 (8b49288) into main (a605bbe) will increase coverage by 16.79%. The diff coverage is 72.00%.

@@             Coverage Diff             @@
##             main      #34       +/-   ##
===========================================
+ Coverage   34.67%   51.47%   +16.79%     
===========================================
  Files           4        4               
  Lines         124      136       +12     
===========================================
+ Hits           43       70       +27     
+ Misses         81       66       -15     

Impacted Files	Coverage Δ
src/K8sClusterManagers.jl	100.00% <ø> (+18.18%)	:arrow_up:
src/native_driver.jl	4.41% <7.14%> (-0.07%)	:arrow_down:
src/pod.jl	97.91% <97.22%> (+47.91%)	:arrow_up:

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[omus]

Marking as breaking as this requires users to update their configure functions