SavedObjects: refactor the export code and move it to the SO client

[pgayvallet]

exportSavedObjectsToStream (src/core/server/saved_objects/export/get_sorted_objects_for_export.ts) has almost been untouched when migrated to KP. We should refactor this to do the following:

• stop exporting it as a static function, and add a new exportSavedObjects or export API to the SO client / repository instead (which would also allow hidden types / custom clients to export their objects)
• get rid of the Boom errors usages
• cleanup and improve the types used in the underlying call chain.

[elasticmachine]

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

[pgayvallet]

This has been addressed when we moved Core code to packages and introduced the @kbn/core-saved-objects-import-export-server-internal package.

Closing