refactor: Payload Generator improvements SO-232

[XVincentX]

This PR revisits a little bit the way we generate the examples from JSON Schemas.

1. Remove the interface+class boilerplate (for a single method) and export a simple function signature somebody can implement
2. Remove the serialisation responsibility from the schema generator — this is something the framework should do somewhere else
3. Refactor the tests to use less snapshots and more assertions

Note: While refactoring the code I also spotted a test passing incorrectly because of a spy that wasn't reset. https://github.com/stoplightio/prism/pull/322/files#diff-e53e98c10a813d2d271cc0fa5c55ddcaR201

[philsturgeon]

This but confuses me. Why was it ‘{}’ before and why is it {} now? They both seem like funny things to happen, don’t we just want an empty string for a header?

[XVincentX]

Good question, I actually wanted to brief/ask you about this anyway.

In OpenAPI 3 documents the schema has a separate property in the Header object (example or examples) — in OAS2 the schema is "melted" with the header definition itself.

For this reason, JSON Schema Faker is kind of confused and generates an empty object for a schema that's not really complete/defined.

I can add a small workaround to standardise the behaviour. Up to you!