Implement unmanaged vFolder and download token API

[kyujin-cho]

This PR implements new unmanaged vFolder API (lablup/backend.ai#74) and download_with_token's token generate API.

[achimnol]

Just a rename please!

[codecov-io]

Codecov Report

Merging #77 into master will increase coverage by 0.66%.
The diff coverage is 42.3%.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   43.29%   43.96%   +0.66%     
==========================================
  Files          47       47              
  Lines        4827     4936     +109     
==========================================
+ Hits         2090     2170      +80     
- Misses       2737     2766      +29

Impacted Files	Coverage Δ
src/ai/backend/client/cli/vfolder.py	28.87% <37.5%> (+0.42%)	⬆️
src/ai/backend/client/vfolder.py	81.85% <50%> (+7.12%)	⬆️
src/ai/backend/client/auth.py	65.45% <0%> (-4.76%)	⬇️
src/ai/backend/client/cli/config.py	43.52% <0%> (-2.19%)	⬇️
src/ai/backend/client/scaling_group.py	32.6% <0%> (-0.21%)	⬇️

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