feat: allow not generating union types' `As...` or `From...`

[jamietanna]

Via https://github.com/oapi-codegen/oapi-codegen/issues/1668

That being said, I'm not sure what the user-facing benefit of this would be, if it's then not possible to retrieve the fields? (unless we have #1680)

This would be a new output option, which would be documented through the JSON Schema.

[jamietanna]

@mweibel how would you be looking at consuming the generated types, if not via the As... methods?

[mweibel]

@jamietanna can you elaborate? We didn't use it before already, so I'm not sure what changed?

[mweibel]

can be closed as #1789 should fix this issue.