[Task Manager] Convert plugin to New Platform

[gmmorris]

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[elasticmachine]

Pinging @elastic/kibana-stack-services (Team:Stack Services)

[gmmorris]

Caught up with @rudolf on the open blockers on moving to New Platform in Task Manager:

Schema & Serializer

SavedObjectsSerializer & SavedObjectsSchema are currently being imported directly out of core. This is still not ideal, and we plan on addressing this across the board later, but is no longer a blocker.

What is a blocker, is that they rely on the exported savedObjectSchemas configuration on uiExports (which is required to support of the task savedObjects in core services), but the plan is not to expose these on kbnServer.uiExports.savedObjectSchemas anymore.
To get around this the plan is to pass this configuration to the SavedObjectsSchema directly, rather than use the current method which uses the kbnServer.uiExports.savedObjectSchemas, which means our SavedObjects client will only know how to interact with task SavedObjects, but that's fine.

SavedObjectRepository

In the coming days we'll hopefully gain access to the repository thank to this PR: #48882

This should allow us to import and initialise our custom Repository.

callCluster

Core now exposes core.elasticsearch.createClient which should cover our custom usage of callCluster, so this should no longer be a blocker.

migrations and mapping

We don't expect these to be available until 7.8 at least, so we'll try and migrate everything to NP except for these and rely on a Legacy plugin that includes these, as this will allow us to unblock other plugins that rely on TM, such as Maps, Lens, Alerting etc.

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Overall, it seems most blockers are about to get unblocked, so the focus next will be:

1. Unblock the usage of Schema & Serializer by addressing how savedObjectSchemas is used.
2. Use core.elasticsearch.createClient to replace how we use callCluster
3. Once the Repository PR is merged, move to using the new core import
4. Move the whole plugin, other than migrations and mapping to NP and unblock other plugins.

Hopefully that'll cover most of our bases, allowing us to derisk as we come closer to v8.0.0