feat: Add Inline Resolver to OpenAPI 2.0 parser

[barryadk]

This change allows allow swagger-parser consumers who are using OpenAPI 2.0 APIs to flatten definition models in the returned spec similar to OpenAPI 3.0 API can be flatten when using swagger-parser 2.x

• Implementation of InlineModelResolver.java and InlineModelResolverTest.java are taken directly from swagger-codegen project. Only modifications are to the package name. This was intentional so all behaviour is consistent.
• Added two additional interfaces in SwaggerParser.java for reading the input spec; these take a ParseOptions object - this is based on the interface used in swagger-parser 2.x