Store ert<->everest realization mapping

[yngve-sk]

Issue
Resolves #9751

Approach
Short description of the approach

(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 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

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: Create Backport PR to latest release

[codspeed-hq]

CodSpeed Performance Report

Merging #9767 will improve performances by 14.34%

_{Comparing yngve-sk:25.01.16.add-everest-realization-info-to-ert (1444308) with main (0b80ac1)}

Summary

⚡ 1 improvements
✅ 23 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	yngve-sk:25.01.16.add-everest-realization-info-to-ert	Change
⚡	test_direct_dark_performance_with_storage[gen_x: 20, sum_x: 20 reals: 10-gen_data_with_obs-get_record_observations]	2.6 ms	2.3 ms	+14.34%