Add support for defining observables in "patch extends"

[rmouritzen-splunk]

Currently observable definitions are flagged with an "unsupported" error for the patch extends case.

A patch extends is triggered when a class or object (most usefully an extension class or object) is defined with an extends property that refers to a class or object in the core schema, and either the name matches the extends or it doesn't have an extends. This patch allows modification of the class or object in the extends property as shown below, and then the patching item disappears (it isn't added to the schema).

A patch currently does these things:

• The profiles in the patching item are merged with the base item.
• The attributes in the patching item are merged with the base item.
• Observable definitions in the patching item cause an error, regardless of kind (top level object observable, attribute observable, or class attribute path observables).

With this issue we want to support merging the observable definitions from the patching item to the base item.

Fixes issue #82.

Related to PR ocsf/ocsf-validator#19.