Support for x-enum-fallback-value field in enums.

[josemonteiro]

If specified, enum will fall back to this value if input value is not part of enum list.

Spec file:

openapi: 3.0
components:
  schemas:
    enum_type_with_fallback:
      type: string
      description: Enum field with fallback value 'unknown'.
      enum:
        - 'a'
        - 'b'
      x-enum-fallback-value: 'unknown'

Generated code:

class EnumTypeWithFallback(Enum):
    a = 'a'
    b = 'b'
    unknown = 'unknown'

    @classmethod
    def _missing_(cls, value):
        return EnumTypeWithFallback.unknown

Usage:

>>> EnumFieldWithFallback("a")
<EnumFieldWithFallback.a: 'a'>

>>> EnumFieldWithFallback("c")
<EnumFieldWithFallback.unknown: 'unknown'>