Add parquet support for seismic

[akustn]

Issue
Resolves #13910

Approach
I approached the issue from the seismic_example, and adjusted the mock to support the usage of .parquet files.
I further adjusted the seismic config and the SeismicObservation to offer .parquet file support.
In addition I added an example to seismic_example that runs with .parquet files and adjusted the README.md accordingly.

(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 screenshots are changed: Review screenshot-PR in ert-testdata,
  merge screenshot-PR in ert-testdata before merging this PR.
• 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 91.91%. Comparing base (57d74db) to head (7b3ae66).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14135      +/-   ##
==========================================
- Coverage   91.92%   91.91%   -0.01%     
==========================================
  Files         482      482              
  Lines       33498    33513      +15     
==========================================
+ Hits        30794    30805      +11     
- Misses       2704     2708       +4     

Flag	Coverage Δ
cli-tests	36.46% <0.00%> (-0.02%)	⬇️
fuzz	44.45% <0.00%> (-0.02%)	⬇️
gui-tests	58.81% <0.00%> (-0.03%)	⬇️
performance-and-unit-tests	80.80% <100.00%> (-0.01%)	⬇️
test	45.97% <0.00%> (-0.03%)	⬇️

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

Files with missing lines	Coverage Δ
src/ert/config/_observations.py	95.61% <100.00%> (+0.07%)	⬆️
src/ert/config/seismic_config.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

---

_{Comparing add-parquet-support (7b3ae66) with main (92ced06)}

[ajaust]

Nice changes!

Minor commit-message suggestions:

• The commit messages switch between parquet and .parquet. Pick one and be consistent. Maybe use parquet when referring to the file format, and reserve  .parquet for the literal file extension.
• Commit subjects can focus more on thevisible change than the implementation location (the actual file); Git already records the affected files. For example, "Support parquet seismic response files in SeismicConfig" is closer to the style used in the project than "Add support for parquet to seismic_config.py". If useful context is needed, add the rationale in the commit body.

[ajaust]

Looks good. Squash the fixups and then feel free to merge. 🎉