Add GCS support

[keigohtr]

What is this PR for?

Add GCS support

This PR includes

• Add GcsHandler
• Add UI for gcs mode
• Add test

What type of PR is it?

Feature

What is the issue?

#69

How should this be tested?

Run unittest

[codecov-io]

Codecov Report

Merging #95 into master will increase coverage by 0.08%.
The diff coverage is 82.75%.

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
+ Coverage   83.69%   83.77%   +0.08%     
==========================================
  Files          42       42              
  Lines        2098     2127      +29     
==========================================
+ Hits         1756     1782      +26     
- Misses        342      345       +3

Impacted Files	Coverage Δ
rekcurd_dashboard/apis/kubernetes_handler.py	82.95% <ø> (ø)	⬆️
rekcurd_dashboard/models/data_server.py	93.18% <100%> (+5.37%)	⬆️
rekcurd_dashboard/apis/api_data_server.py	95.39% <80.76%> (-3.02%)	⬇️

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[yuki-mt]

LGTM!