[feat] enable gke clusters from existing

[shaneutt]

• chore: move gke var names into gke pkg
• chore: refactor gke auth into discreet fn
• feat: enable gke clusters from existing

Unblocks work going on in Kong/kubernetes-ingress-controller#1095

[codecov-commenter]

Codecov Report

Merging #67 (380a640) into main (b38b45c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #67   +/-   ##
=======================================
  Coverage   22.09%   22.09%           
=======================================
  Files           2        2           
  Lines         172      172           
=======================================
  Hits           38       38           
  Misses        133      133           
  Partials        1        1           

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