Allow "unresolvable conflict" errors to be circumvented using initialNamespaces

[jportner]

Blocked by: #102957

In #101904 (PR: #102585), we added support for initialNamespaces to be used when creating isolated object types. This allows consumers to create any non-space-agnostic object and define the initial space(s) it should be created in. It breaks down the barriers between spaces in the SavedObjectsRepository (SOR) and SavedObjectsClient (SOC), where operations are otherwise generally isolated to a single space.

However, multi-namespace objects currently have a limitation because they have globally unique IDs: if you try to create an object in space A, but an object with the same ID already exists in space B, that operation will fail with an "unresolvable conflict" error. This makes sense for a normal create operation, but the assumption breaks down when using the initialNamespaces option.

When creating an object and using the initialNamespaces option in conjunction with overwrite: true, the consumer is being deliberate about what they want to accomplish, and we should allow them to overwrite the existing object without throwing an unresolvable conflict error. However, we should only allow this if the user has privileges to do all of the following:

• Create in the current space
• Create in the target spaces (defined by initialNamespaces)
• Create in at least one space where the object currently exists

To enable this behavior, we have to do two things:

1. Conduct preflight checks in the Repository for create and bulkCreate to see if the object currently exists, and if so, what space(s) it exists in
2. Avoid throwing the unresolvable conflict error in the SOR iff initialNamespaces and overwrite: true are both specified