Add support for deep oneOf inheritance

[glopesdev]

This PR expands discriminator inheritance resolution to support deep inheritance hierarchies. It also resolves several complicated assumptions found in NJsonSchema that would limit future support for complex inheritance schemes.

The core of the approach is to resolve oneOf inheritance in two passes:

1. for each discriminator schema:
   a. if schema is inline, move the discriminator to a separate type definition
   b. if the discriminator has oneOf declarations, inject the discriminator as allOf schema in each of the derived schemas
2. remove discriminator properties from all derived schemas

A simple regression test for a deep inheritance schema is added, but more extensive tests will be required in the future to ensure all edge cases are covered.