dio 5.5.0

What's new

• Raise the min Dart SDK version to 2.18.0.
• Add constructor for DioExceptionType.badCertificate.
• Create type alias DioMediaType for http_parser's MediaType.
• Fix the type conversion regression when using MultipartFile.fromBytes.
• Split the Web implementation to package:dio_web_adapter.
• Add FusedTransformer for improved performance when decoding JSON.
• Set FusedTransformer as the default transformer.
• Improves InterceptorState.toString().
• If the CancelToken got canceled before making requests,
  throws the exception directly rather than cut actual HTTP requests afterward.
• Catch MediaType parse exception in Transformer.isJsonMimeType.
• Improves warning logs on the Web platform.
• Improves memory allocating when using CancelToken.