Cache realization states to avoid redundant checking of files

[yngve-sk]

(this is a draft that shows somewhat what it would look like if we added state monitoring, currently there is quite some room for refactoring/improvement in readability/more tests)

Example of the state:

self.to_dataframe()
                                                      value     timestamp
realization kind                key                                      
0           SNAKE_OIL_PARAM_BPR BPR_138_PERSISTENCE    True  1.718870e+09
                                BPR_555_PERSISTENCE    True  1.718870e+09
            SNAKE_OIL_PARAM_OP1 OP1_DIVERGENCE_SCALE   True  1.718870e+09
                                OP1_OCTAVES            True  1.718870e+09
                                OP1_OFFSET             True  1.718870e+09
                                OP1_PERSISTENCE        True  1.718870e+09
            SNAKE_OIL_PARAM_OP2 OP2_DIVERGENCE_SCALE   True  1.718870e+09
                                OP2_OCTAVES            True  1.718870e+09
                                OP2_OFFSET             True  1.718870e+09
                                OP2_PERSISTENCE        True  1.718870e+09
            TOP                 TOP                    True  1.718870e+09

TOP->TOP indicates that the parameter TOP was updated (no groups here), kind->key otherwise indicates RESPONSE TYPE -> RESPONSE KEY / PARAMETER GROUP -> PARAMETER KEY

Benchmarking (Testing on RGS on large plot case)

• Migrations took about 10 minutes
• Opening with combined datasets takes ~25 minutes when migrations are done
• Computing the state takes 1m~1m30s per ensemble
• Opening case with pre-combine ert takes ~30 sec
• Opening case with this PR takes 2m26s, but 2m2s is spent re-computing the state, making it about the same if not slightly faster than pre-combine if we just store the state in a file.

Issue Resolves #8125

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.10778% with 13 lines in your changes missing coverage. Please review.

Project coverage is 86.97%. Comparing base (7281a93) to head (15fe79f).

Files	Patch %	Lines
src/ert/storage/realization_state.py	93.64%	11 Missing :warning:
src/ert/storage/local_ensemble.py	98.72%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #8135 +/- ## ========================================== + Coverage 86.86% 86.97% +0.10% ========================================== Files 379 380 +1 Lines 23971 24293 +322 Branches 620 624 +4 ========================================== + Hits 20822 21128 +306 - Misses 3068 3086 +18 + Partials 81 79 -2 ```

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