feat: Add command 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]

Codecov Report

Merging #165 (fd5e6dd) into main (4899ed3) will decrease coverage by 0.07%.
The diff coverage is 35.61%.

@@            Coverage Diff             @@
##             main     #165      +/-   ##
==========================================
- Coverage   41.81%   41.74%   -0.08%     
==========================================
  Files          68       69       +1     
  Lines        6613     6684      +71     
==========================================
+ Hits         2765     2790      +25     
- Misses       3848     3894      +46     

Impacted Files	Coverage Δ
src/ai/backend/client/cli/resource.py	28.88% <28.88%> (ø)
src/ai/backend/client/cli/pretty.py	43.57% <33.33%> (-0.96%)	⬇️
src/ai/backend/client/func/resource.py	51.89% <50.00%> (-0.35%)	⬇️
src/ai/backend/client/cli/manager.py	28.87% <66.66%> (+0.50%)	⬆️
src/ai/backend/client/cli/__init__.py	100.00% <100.00%> (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 4899ed3...fd5e6dd. Read the comment docs.

[achimnol]

There may be more resource slot types.
Please use session.Resource.get_resource_slots() to retrieve the exact list of the currently configured resource slots, and then iterate over it when displaying the remainings.
Note that you may need to have an extra mapping from the resource slot name to a human-readable name with a fallback to the raw slot name.