feat: Add API to get available resources

[romanceformoon]

lablup/backend.ai#249

This PR adds API to get available resources in groups. Users can get available resources by API GET method.

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

Merging #442 (0805344) into main (27d9325) will decrease coverage by 0.12%. The diff coverage is 34.21%.

@@            Coverage Diff             @@
##             main     #442      +/-   ##
==========================================
- Coverage   48.28%   48.16%   -0.13%     
==========================================
  Files          52       52              
  Lines        8708     8763      +55     
==========================================
+ Hits         4205     4221      +16     
- Misses       4503     4542      +39     

Impacted Files	Coverage Δ
src/ai/backend/manager/server.py	60.98% <ø> (ø)
src/ai/backend/manager/models/resource_preset.py	60.34% <34.21%> (-12.74%)	:arrow_down:
src/ai/backend/manager/idle.py	30.84% <0.00%> (-0.81%)	:arrow_down:

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