[FEATURE] String format type mapping

[Matej-Hlatky]

Is your feature request related to a problem? Please describe.

Have option to specify String type mapping based on "format" value to custom Dart types, like uuid, Uri, Uint8List etc.

Describe the solution you'd like

Be able to specify type mapping via options in build.yaml file, similar to this:

targets:
  $default:
    builders:
      swagger_dart_code_generator:
        options:
          scalars:
            uuid:
              type: uuid
              from_string_function_name: uuidFromString
              to_string_function_name: uuidToString
              import_url: package:uuid/uuid.dart

These functions would need to be writen by developer in some specific Dart file.

The inspiration is from graphql_codegen.