Refine app proxy support

[achimnol]

In the progress of lablup/backend.ai-agent#189.

[codecov]

Codecov Report

Merging #90 into master will decrease coverage by 0.04%.
The diff coverage is 14.81%.

@@            Coverage Diff            @@
##           master     #90      +/-   ##
=========================================
- Coverage   42.55%   42.5%   -0.05%     
=========================================
  Files          56      56              
  Lines        5384    5397      +13     
=========================================
+ Hits         2291    2294       +3     
- Misses       3093    3103      +10

Impacted Files	Coverage Δ
src/ai/backend/client/cli/app.py	23.92% <14.81%> (-0.06%)	⬇️

---

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...007f8bf. Read the comment docs.