[meta] Kibana managed upgrades

[pgayvallet]

This is a meta issue to track the work required for managed upgrades

First phase

• Create the 'migrator' node role #150295
• Create the modelVersion API and types #150297
• DocumentMigrator: support downward migrations #150301
• Implement zero downtime migration algorithm #150309
• Adapt the SOR for managed upgrades #150312
• [ZDT] migration algorithm follow-up #161067

Second phase

• Implement lossless up/down migration #150314
• Add rollback support #150315

Testing plan for ZDT migrations

• [tbd]

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)