Refactor rollover state persistence

[lferran]

Description

Before, the rollover state was kept in the extra parameter of the Shards protobuf.
Now that we want to rollover not only internal index node shards, but also external indexes, we need to decouple this logic from the shards object.

How was this PR tested?

Integration, unit and local tests.

[codecov]

Codecov Report

Attention: Patch coverage is 85.91549% with 10 lines in your changes missing coverage. Please review.

Project coverage is 86.59%. Comparing base (adb1515) to head (18869cd).
Report is 1 commits behind head on main.

Files	Patch %	Lines
nucliadb/src/nucliadb/common/cluster/rollover.py	80.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2395      +/-   ##
==========================================
- Coverage   86.60%   86.59%   -0.01%     
==========================================
  Files         373      373              
  Lines       22874    22902      +28     
==========================================
+ Hits        19811    19833      +22     
- Misses       3063     3069       +6     

Flag	Coverage Δ
nucliadb	73.50% <85.91%> (-0.01%)	⬇️
nucliadb-ingest	29.66% <19.71%> (-0.02%)	⬇️
nucliadb-reader	25.37% <19.71%> (-0.01%)	⬇️
nucliadb-search	37.49% <19.71%> (-0.03%)	⬇️
nucliadb-standalone	47.79% <19.71%> (-0.05%)	⬇️
nucliadb-train	46.71% <19.71%> (-0.05%)	⬇️
nucliadb-writer	40.66% <19.71%> (-0.04%)	⬇️
nucliadb_dataset	60.09% <ø> (ø)
nucliadb_models	85.11% <ø> (ø)
nucliadb_sdk	77.48% <ø> (ø)
nucliadb_sidecar	94.44% <ø> (ø)
nucliadb_telemetry	86.55% <ø> (ø)
nucliadb_utils	83.22% <ø> (ø)

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