[ZDT Migration] support updating mapping for root fields

[pgayvallet]

atm the zdt migration algorithm fully ignores the root field mappings after the initial index creation, and never update them. The additive mapping changes are only based on the SO types that have new model versions:

kibana/packages/core/saved-objects/core-saved-objects-migration-server-internal/src/zdt/model/stages/init.ts

Lines 165 to 169 in 79b5754

	const additiveMappingChanges = generateAdditiveMappingDiff({
	types,
	meta: currentMappings._meta ?? {},
	deletedTypes: context.deletedTypes,
	});

kibana/packages/core/saved-objects/core-saved-objects-migration-server-internal/src/zdt/utils/generate_additive_mapping_diff.ts

Lines 26 to 49 in 9cddb60

	/**
	* Generates the additive mapping diff we will need to update the index mapping with.
	*
	* @param types The types to generate the diff for
	* @param meta The meta field of the index we're migrating
	* @param deletedTypes The list of deleted types to ignore during diff/comparison
	*/
	export const generateAdditiveMappingDiff = ({
	types,
	meta,
	deletedTypes,
	}: GenerateAdditiveMappingsDiffOpts): SavedObjectsMappingProperties => {
	const typeVersions = getVirtualVersionMap(types);
	const mappingVersion = getVirtualVersionsFromMappingMeta({
	meta,
	source: 'mappingVersions',
	knownTypes: types.map((type) => type.name),
	});
	if (!mappingVersion) {
	// should never occur given we checked previously in the flow but better safe than sorry.
	throw new Error(
	'Cannot generate additive mapping diff: mappingVersions not present on index meta'
	);
	}

More important, the version check we're performing to check if we do need to update the mappings is also ignoring the root field mappings, and is only based on comparing the SO type model versions:

kibana/packages/core/saved-objects/core-saved-objects-migration-server-internal/src/zdt/model/stages/init.ts

Lines 149 to 154 in 79b5754

	const versionCheck = checkVersionCompatibility({
	mappings: currentMappings,
	types,
	source: 'mappingVersions',
	deletedTypes: context.deletedTypes,
	});

We need to update this logic to support updating our mappings when new root fields are introduced.

This could be done by checking the diff in mappings between the index's and the app's expected mappings (as v2 is doing), or this could be performed by introducing a coreVersion in the model version's meta stored in the index.

[elasticmachine]

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