(feature): Add support for x-fern-encoding

[amckinney]

This adds support for the x-fern-encoding extension. For now, we only support this extension on the types where they're most commonly used (e.g. map and oneof):

components:
  schemas:
    Metadata:
      type: object
      additionalProperties:
        $ref: '#/components/schemas/MetadataValue'
      x-fern-encoding:
        proto:
          type: google.protobuf.Struct
    MetadataValue:
      nullable: true
      oneOf:
        - type: number
          format: double
        - type: string
        - type: boolean
        - type: array
          items:
            $ref: '#/components/schemas/Value'
      x-fern-encoding:
        proto:
          type: google.protobuf.Value
    Value:
      oneOf:
        - type: number
          format: double
        - type: string
        - type: boolean

This also makes a slight adjustment to the IR generator so that we consider creating a ProtobufType for types that specify an encoding without a source (which is particularly relevant for users that specify x-fern-encoding).