Various Enum Fixes + Tweaks

[gbdubs]

• Adds an enum prefix for every OpenAPI enum type, to avoid collisions + clean up the namespace (otherwise constants like api.En are generated).
• Removes use of YAML feature I hadn't understood prior (anchors + links), putting everything into the schemas, since definitions look like they're a Swagger (OAPI2.0) construct.
• Adds some basic Enum Round-Trip unit tests to pactasrv/conv
• removes usage of pacta.ParseX functions within the conv package, since these are less reliable than using the one for one, switch driven converters.