Support custom primitives with arbitrary formats

[susliko]

Currently SwaggerPrimitive is defined as

class SwaggerPrimitive[Typ <: SwaggerValue](
    val format: Option[OpenApiFormat[Typ]] = None,
    ...
) 

where OpenApiFormat is a sealed trait. This means that one cannot define custom primitives with arbitrary formats whilst the OpenAPI Data Types spec states:

However, format is an open value, so you can use any formats, even not those defined by the OpenAPI Specification,
...
Tools can use the format to validate the input or to map the value to a specific type in the chosen programming language. Tools that do not support a specific format may default back to the type alone, as if the format is not specified.