[Actions] relatedSavedObjects should allow multiple namespaces

[ymao1]

    🤔 we have a discrepancy here. The actions plugin API [only contemplates a single `namespace: string`](https://github.com/elastic/kibana/blob/main/x-pack/plugins/actions/server/lib/related_saved_objects.ts#L15) for each of the "relatedSavedObjects". Isn't there a way that you can choose the most relevant one? 😝 

A possible "solution" would be for the notifications plugin to split the related SO into multiple related SOs if they belong to multiple spaces.
On the other hand, we could update the schema of RelatedSavedObjects, but I'm afraid this would have a bunch of side effects that we'd have to cope with.

Originally posted by @gsoldevila in #143303 (comment)

When we created the idea of relatedSavedObjects to be passed to the action executor and persisted to the event log, saved objects could only exist in a single space so we had a namespace field. Now we have a need to persist saved objects that can exist in multiple spaces so we need a way to store multiple namespaces.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[ymao1]

Closing as not needed.