Implement dotfiles API

[kyujin-cho]

This PR implements Dotfile API from lablup/backend.ai-manager#220

[codecov]

Codecov Report

Merging #85 into master will decrease coverage by 0.19%.
The diff coverage is 36.53%.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
- Coverage   42.55%   42.36%   -0.20%     
==========================================
  Files          56       58       +2     
  Lines        5384     5538     +154     
==========================================
+ Hits         2291     2346      +55     
- Misses       3093     3192      +99     

Impacted Files	Coverage Δ
src/ai/backend/client/session.py	93.65% <0.00%> (+0.12%)	⬆️
src/ai/backend/client/func/dotfile.py	36.53% <0.00%> (ø)
src/ai/backend/client/cli/dotfile.py	31.95% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update afe8d39...114c59d. Read the comment docs.