Fix ref handling in CB, AnyOf & examples (fixes : #2086)

[anthochristen]

This PR is to address the $ref handling issues in callbacks and discriminator mapping objects as detailed in #2086. It does it with the following changes

• Extends discriminator mapping handling for anyOf like its done for OneOf in SchemaProcessor class.
• Add ref path resolution for examples of Parameters
• Handles ref path resolution for array items with ComplexSchemas
• Refactors the processRefToExternalPathItem in ExternalRefProcessor so that it can be used for $ref path resolution in PathItems used within callbacks.

Each done in dedicated commits, although there might be some spill over.

[anthochristen]

@gracekarina I see that you have worked on the files in which most of this changes are done for this issue (#2086). Would be great if you can take a look at these changes.

[anthochristen]

Have a blanket IT type test for now, can add more in depth UTs once I get some comments on these changes.

[anthochristen]

Thanks @frantuma for assigning this to yourself!

[anthochristen]

P.S: These issues are not present in the npm swagger-parser lib.

[anthochristen]

@frantuma I've changed the oneOf and anyOf to be inclusive, kindly help take a look when possible, thanks!