Add an `isJson` getter to `MediaType`

[kevmoo]

It's so crazy common to want to check this – and it's not super straight-forward.

See

https://github.com/google/googleapis.dart/blob/ec7b27cf3895c92b75ef407e1c109a9f7eba3a72/discoveryapis_commons/lib/src/utils.dart#L10-L16

https://github.com/GoogleCloudPlatform/functions-framework-dart/blob/80da6491efbf2e58a2c3f4732a3a7f055fe5eb37/functions_framework/lib/src/json_request_utils.dart#L39