[ZDT] create aliases during initial index creation

[pgayvallet]

At the moment, we're creating the index and and aliases in different steps, respectively CREATE_TARGET_INDEX and UPDATE_ALIASES.

This was done for simplicity's sake, to keep the index update logic into a single step, given we want to also check/update alias for subsequent migrations (when the indices are already present)

However, for performance's sake, we should now try to optimize our workflow, and have the create operation performed during CREATE_TARGET_INDEX also create the aliases, to avoid one unecessary roundtrip.

Tech pointers

Generating the alias actions that are exposed on state.aliasActions:

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

Lines 110 to 116 in 98efb6a

	const existingAliases = Object.keys(indices[currentIndex].aliases);
	const aliasActions = getAliasActions({
	existingAliases,
	currentIndex,
	indexPrefix: context.indexPrefix,
	kibanaVersion: context.kibanaVersion,
	});

The action that should now use those alias actions:

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

Lines 64 to 70 in 53173f1

	CREATE_TARGET_INDEX: (state: CreateTargetIndexState) =>
	Actions.createIndex({
	client,
	indexName: state.currentIndex,
	mappings: state.indexMappings,
	esCapabilities: context.esCapabilities,
	}),

Note that getAliasActions technically can return any kind of AliasAction (including deletions), even if today we're only doing additions (and we will likely only do addition for index creation anyway), but we still need some kind of logic to decide if we can extract the alias names from the AliasActions list

[elasticmachine]

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