[Migrations v2] provide a failure_strategy hook in SO registry for migrations

[Bamieh]

Some saved objects registries are OK with dropping their documents if the mgirations failed. Like a few collectors SOs and tasks SOs.

Providing a failure_strategy to the migrations to be triggered when migrations fail to allow pluginis to specify what to do on faiilure. This might reduce the amount of friction when migrations fail and reduce the number of upgrade failures.

One common strategy is to skipOnMigrationFailiure. We can provide a few helper functions to automatically do that on faliure.

Other custom implementations in mind is ammending the doc and trying again or allowing to drop some objects but not others.

An example usage for the feature is something like this:

import { skipOnMigration } from 'src/core/server'

savedObjects.register({
  ...
  failure_strategy: skipOnMigrationFailiure
})

[elasticmachine]

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

[pgayvallet]

with serverless migrations, and the fact that data transformations were significantly reduced, I don't think we would need this anymore, especially given it would only partially solve our problems (any type not optin for this behavior would still cause issues).

So I'll go ahead and close this, feel free to reopen if necessary