Migrate observation report to parquet and make it as blob artifact

[xjules]

Issue
Resolves #13484

Approach
Ensemble storage will contain a blobs folder to contain different update related artefacts.
This migrates observation report into the blob.

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.94%. Comparing base (d5f13f9) to head (61a35ef).
⚠️ Report is 10 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13495      +/-   ##
==========================================
- Coverage   89.99%   89.94%   -0.06%     
==========================================
  Files         460      461       +1     
  Lines       32320    32392      +72     
==========================================
+ Hits        29087    29134      +47     
- Misses       3233     3258      +25     

Flag	Coverage Δ
cli-tests	37.22% <100.00%> (+<0.01%)	⬆️
fuzz	44.08% <100.00%> (+0.01%)	⬆️
gui-tests	66.88% <100.00%> (-0.05%)	⬇️
performance-and-unit-tests	78.06% <100.00%> (-0.01%)	⬇️
test	45.62% <62.96%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/ert/analysis/_enif_update.py	96.19% <ø> (ø)
src/ert/analysis/_es_update.py	94.40% <ø> (ø)
src/ert/analysis/event.py	98.07% <100.00%> (ø)
src/ert/run_models/update_run_model.py	98.18% <100.00%> (+0.26%)	⬆️
src/ert/storage/blob_data.py	100.00% <100.00%> (ø)
src/ert/storage/local_ensemble.py	96.12% <100.00%> (+<0.01%)	⬆️

... and 8 files with indirect coverage changes

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

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_{Comparing xjules:obs_report_blob (61a35ef) with main (b4ed493)}