chore: refactor configs for easy volume mounting

[nkoppisetty]

Explain what this PR does.

1. Moved _config.py out of config directory for easy ConfigMap mounting
2. Renamed default configs and metric names according to standardized names
3. Added app and rollouts_pod_template_hash as labels for default rollout metrics
4. Fixed postprocess - removed hardcoded labels
5. Fixed trainer to work with rollouts_pod_template_hash label
6. Fixed and added more test cases

[codecov-commenter]

Codecov Report

Merging #100 (c7877c5) into main (181c353) will decrease coverage by 0.05%. The diff coverage is 85.00%.

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
- Coverage   85.93%   85.89%   -0.05%     
==========================================
  Files          19       18       -1     
  Lines         967      971       +4     
  Branches      100      105       +5     
==========================================
+ Hits          831      834       +3     
  Misses        115      115              
- Partials       21       22       +1     

Impacted Files	Coverage Δ
numaprom/tools.py	84.02% <33.33%> (-0.09%)	:arrow_down:
numaprom/udf/postprocess.py	95.50% <85.71%> (-1.01%)	:arrow_down:
numaprom/__init__.py	85.00% <100.00%> (+1.66%)	:arrow_up:
numaprom/_config.py	100.00% <100.00%> (ø)
numaprom/_constants.py	100.00% <100.00%> (ø)
numaprom/udf/inference.py	100.00% <100.00%> (ø)
numaprom/udsink/train_rollout.py	84.48% <100.00%> (+0.41%)	:arrow_up:

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