Do not scan_keys on getting labelsets

[lferran]

Description

Current implementation of getting labelsets uses tikv scan keys to get the list of labelsets.
Tikv doesn't behave very well when scanning keys.

This PR does:

• materializes the list of labelsets in a separate key in the KV
• makes sure to update the list on additions and deletions
• adds a migration step to materialize labelset lists for existing kbs
• runs backward compatible code to make sure the feature works while the migration hasn't run yet

How was this PR tested?

Integration tests

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f5f1bd2) 84.35% compared to head (bdf16c5) 84.34%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
nucliadb/nucliadb/ingest/orm/knowledgebox.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1863      +/-   ##
==========================================
- Coverage   84.35%   84.34%   -0.02%     
==========================================
  Files         328      328              
  Lines       18757    18747      -10     
==========================================
- Hits        15823    15812      -11     
- Misses       2934     2935       +1     

Flag	Coverage Δ
ingest	69.74% <50.00%> (+0.01%)	⬆️
sdk	87.85% <ø> (ø)
utils	81.81% <ø> (ø)

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[lferran]

[sc-8994]

[shortcut-integration]

This pull request has been linked to Shortcut Story #8994: reduce tikv scans.