[Saved Objects Client] Allow sorting multiple types in the same find request

[afharo]

At the moment, it is not possible to sort the results of 2 Saved Object types in the same find request if the sorting field is not a root field.

kibana/src/core/packages/saved-objects/api-server-internal/src/lib/search/search_dsl/sorting_params.ts

Lines 46 to 52 in b289a29

	if (types.length > 1) {
	const rootField = getProperty(mappings, sortField);
	if (!rootField) {
	throw Boom.badRequest(
	`Unable to sort multiple types by field ${sortField}, not a root property`
	);
	}

I believe that the underlying reason is that, even if the field name is the same (i.e.: name.keyword), they are actually persisted in ES as different fields (<type 1>.name.keyword, <type 2>.name.keyword), so sorting would be inaccurate.

I wonder if we could work around this limitation on ES by adding a runtime field that merges all fields

# Pseudo code
runtime_field.name.keyword = <type 1>.name.keyword ?? <type 2>.name.keyword

Then, we sort via the runtime field.

Use cases

This is mostly required by those SOs that needed to be duplicated when moving from agnostic to multi-space: #221568

[elasticmachine]

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