Add commands to get PipelineRun and TaskRun logs in to k8s tree

[evidolob]

This PR adds two commands Show Log and Follow Logs in kubernetos tree for PipelineRun and TaskRun items:

Note: Getting logs implemented with tkn cli (exactly the dame as for Tekton tree), it is possible to fetch logs using pure kubernetos API but it requires a big effort, basically we need to re implement part of tkn CLI: https://github.com/tektoncd/cli/blob/6dad772d2bd697c0f66ec404a4db73b449b2cfa3/pkg/log/task_reader.go#L101
https://github.com/tektoncd/cli/blob/6dad772d2bd697c0f66ec404a4db73b449b2cfa3/pkg/log/pipeline_reader.go#L85

Fix: #2

[codecov-io]

Codecov Report

Merging #247 into master will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #247      +/-   ##
==========================================
+ Coverage   71.15%   71.33%   +0.18%     
==========================================
  Files          37       38       +1     
  Lines        2368     2383      +15     
  Branches      397      401       +4     
==========================================
+ Hits         1685     1700      +15     
  Misses        683      683              

Impacted Files	Coverage Δ
src/extension.ts	89.71% <100.00%> (+0.09%)	⬆️
src/kubernetes.ts	100.00% <100.00%> (ø)

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