[App Arch] [Cleanup] Saved object migrations from kibana plugin to new platform

[alexwizp]

Summary

This PR fixed problem described in #59044 (comment)

Originally posted by @tylersmalley in #59044

Checklist

Delete any items that are not applicable to this PR.

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[alexwizp]

I think we need to add two generics cause it allow us to avoid a lot of casting in future.

Like e.g. here: src/plugins/dashboard/server/saved_objects/dashboard_migrations.ts
Also using this approach we can guarantee that all Doc's extended from SavedObjectUnsanitizedDoc or SavedObjectSanitizedDoc

[pgayvallet]

To follow our slack discussion:

This is an alternative of #63943

However on #63943 we choose to only generify the attributes property of SavedObjectUnsanitizedDoc, as we didn't see any (valid) reasons plugin migration functions should be able to superseed/override the root SO type. This could also be dangerous as it may cause developer errors trying to push changes/properties not defined in the schema.

Do you have any usages where you would need to use type override on the whole SavedObjectUnsanitizedDoc type?

[alexwizp]

At least in version 7.0.0 references were added. And we should somehow handle it.
Also who knows what will be added in future?

From the other hand if someone wants to add something into doc we have at least 2 options: use JS without typings, just cast object to needed type =) So, validation should be implemented in a different way

[rudolf]

Currently there's an index signature so anything goes, but we should definitely remove that

kibana/src/core/server/saved_objects/serialization/types.ts

Line 63 in 97d1685

[rootProp: string]: any;

Root properties cannot be added by plugins, so if we ever add them in Core, we can update this type at the same time like we recently did for the new namespaces root property.

[pgayvallet]

@alexwizp could I get your review on #63943? Any argument on why the PR wouldn't answer the need in a better/safer way is welcome.

[kibanamachine]

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