Provide results when Saved Objects are shared or unshared

[jportner]

Currently, when sharing or unsharing saved objects (via the Saved Objects Client's addToNamespaces and deleteFromNamespaces methods), the result is an empty object. This is because all planned consumers did not need any additional information; there is precedent for this, as the delete method works the same way.

However, when starting work on #54836, it became clear that we need to be able to identify what namespaces an object exists in after the operation completes to generate the audit events with the proper level of detail.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)