Implement zero downtime migration algorithm

[pgayvallet]

Part of #150296
Somehow depends on #150301 (could maybe to both in parallel, but doing the Doc migration part first would help us having a better vision probably)

• add a new migrations.managed configuration setting to switch between the two implementations
• implement the base of the algorithm, especially the part storing / checking the model version in the index's meta
  • will unblock Adapt the SOR for managed upgrades #150312
• implement the whole migration algorithm

The new managed algorithm will be dissociated from the existing (v2) one, but ideally we would reuse as much low-level blocks as possible (e.g actions)

Follow-up / optimizations:

• [req] support for upgrading docs based on coreMigrationVersion
• [req] support for executing only the index-state part when during from non-migrator instances (to bootstrap the index)
• [opt] during the reindex following the update of the mapping, only reindex document that got their mappings updated
• [opt] optimize documentsUpdateInit to perform a version check and skip most tasks if same version
• [opt] create the aliases directly during CREATE_TARGET_INDEX to allow skipping UPDATE_ALIASES when creating the index

PRs

• Bootstrap ZDT migration algorithm #151282
• Implement first stages of the ZDT migration algorithm #152219
• Implement the second part of the ZDT migration algorithm #153031