Allow hidden saved-objects to be imported/exported

[kobelb]

It's becoming more and more common for developers to make saved-objects hidden and write a custom client that consumers must use when interacting with these saved-objects. This restricts consumers from using the standard SavedObjectsClient and HTTP APIs to interact with these saved-objects. This is currently being done for a number of reasons:

• Implement custom authorization
• Enforce validation on the saved-object's attributes
• Create API Keys that are stored using encrypted saved-object attributes

However, there are drawbacks to doing so and these saved-objects can no longer be imported or exported using saved-object management's import/export.

Potential Solutions

Support custom-logic for import/export

This would allow the plugins that declare a saved-object as being hidden to write custom logic that would be executed for the saved-object to support import/export. I'm not exactly sure how this would work practically, and further exploration on the specifics of this are required

Use-case specific import/export

It's also possible for us to not try to solve this at that platform level, and require a use-case specific import/export. This would require that every time a saved-object is marked as hidden that a purpose-built import/export UI and API be implemented. This would create a rather fragmented experience, and make it difficult for end-users to import/export all of their saved-objects.

[elasticmachine]

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

[kobelb]

Dupe of #82027