[eval] CI: compare eval run against stored baseline

[thorrester]

Part of #380
Depends on: #383, #384

What

Logic for CI workflows where a team runs evaluations on the current version and compares against a previously stored run.

• Python API: compare(current_result, baseline) where baseline can be specified as:
  • Service version (e.g. service_version="1.2.0")
  • Space/name/version triple
  • Specific eval run uid
• Server route to fetch a stored eval result by the above identifiers
• Comparison output: per-metric delta, pass/fail regression verdict
• CLI: opsml eval compare --baseline-version 1.2.0 for use in CI scripts
• Should return a non-zero exit code on regression (for CI gate use)