Created the test plugin with basic functionality. Subsequent commits
will demonstrate individual steps that need to be taken, as described
by the Sharing Saved Objects developer guide.

Note: this simple test plugin doesn't use links to other object types,
so Step 1 is skipped intentionally.

This step demonstrates the changes to update client code to use the new
SavedObjectsClient `resolve()` method instead of `get()`.

This step demonstrates the changes to update client code to correctly
handle the three different `resolve()` outcomes. It adds an optional
dependency on the Spaces plugin, and uses the SpacesApi to change the UI
if necessary.

This step demonstrate the changes to update the saved object type
registration, which will cause this isolated object type to be converted
to become share-capable during the 8.0 upgrade process.

Note: the previous steps can be backported to 7.x, but this step cannot
be backported, because the conversion cannot take place before 8.0.