feature: add support for multi-arch image

[kyujin-cho]

This PR resolves lablup/backend.ai#374.

What's new?

• move image metadata storage backend to postgreSQL
• apply ORM pattern for new images and image_aliases table
• add support for multi-architecture image handling
• collect agent machine's CPU architecture
• consider image architecture when scheduling kernel to prevent container starting with invalid architecture

Additional works needed

• generalize ORM declaration and DB session lifecycle (this can be handled on seperate issue/PR)
• alias etcd *-image command to newly added image * command, for backward compatibility
• implement readonly txns and advisory lock (additional talks needed) for AsyncSession

[codecov]

Codecov Report

Merging #541 (2fbfe6f) into main (3e444e0) will increase coverage by 0.29%.
The diff coverage is 41.59%.

❗ Current head 2fbfe6f differs from pull request most recent head 39198cb. Consider uploading reports for the commit 39198cb to get more accurate results

@@            Coverage Diff             @@
##             main     #541      +/-   ##
==========================================
+ Coverage   49.07%   49.36%   +0.29%     
==========================================
  Files          55       55              
  Lines        9166     8945     -221     
==========================================
- Hits         4498     4416      -82     
+ Misses       4668     4529     -139     

Impacted Files	Coverage Δ
src/ai/backend/manager/api/manager.py	37.26% <0.00%> (-0.63%)	⬇️
src/ai/backend/manager/models/session_template.py	31.64% <ø> (ø)
src/ai/backend/manager/scheduler/dispatcher.py	24.93% <12.50%> (-0.47%)	⬇️
src/ai/backend/manager/registry.py	17.70% <16.66%> (-0.03%)	⬇️
src/ai/backend/manager/cli/image_impl.py	23.15% <23.15%> (ø)
src/ai/backend/manager/models/image.py	49.50% <43.39%> (-8.29%)	⬇️
src/ai/backend/manager/cli/etcd.py	62.43% <51.72%> (+12.64%)	⬆️
src/ai/backend/manager/models/utils.py	78.01% <52.38%> (-4.49%)	⬇️
src/ai/backend/manager/models/base.py	53.80% <91.66%> (+0.26%)	⬆️
src/ai/backend/manager/cli/__main__.py	53.76% <100.00%> (+1.01%)	⬆️
... and 8 more

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[achimnol]

First batch of reviews. Also please merge the main branch.

[achimnol]

Good job! 🎉
LGTM for now, but there may be more future improvements of course.
Let's handle them as separate issues and PRs.