[Platform] Add import/export API to core SO service

[pgayvallet]

Need to add the import/export APIs that were exposed in the legacy SO service to the core SO service, as the spaces plugin uses them.

kibana/src/legacy/server/saved_objects/saved_objects_mixin.js

Lines 96 to 101 in 8524303

	importExport: {
	objectLimit: server.config().get('savedObjects.maxImportExportSize'),
	importSavedObjects,
	resolveImportErrors,
	getSortedObjectsForExport,
	},

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[pgayvallet]

Except from the objectLimit that needs an accessors, all other methods are static utility that don't have any context, and could be just exported from src/core/server. Not sure if we prefer that, or to add them to the SO start contract?

[rudolf]

I'm 👍 for just exposing these as static utilities.