fix(ferry): Properly type-constrain ErrorTypedLink's forward response stream

[jimgerth]

In the ErrorTypedLink, the response stream recieved via the forward function is not type-constrained, thus cast it to be of the generic types TData and TVars.

This fixes an error that could sometimes occur, where the forward response stream was of a different type than TData and TVars, which mismatched with the StreamTransformer's inferred types, throwing an exception.

LinkException(type '_StreamHandlerTransformer<OperationResponse<dynamic, dynamic>, OperationResponse<dynamic, dynamic>>' is not a subtype of type 'StreamTransformer<OperationResponse<ConcreteTypeData, ConcreteTypeVars>, OperationResponse<dynamic, dynamic>>' of 'streamTransformer',
#0      Stream.transform (dart:async/stream.dart:1007:50)
#1      ErrorTypedLink.request (package:ferry/src/error_typed_link.dart:25:41)
#2      _TypedLinkChain.request.<anonymous closure>.<anonymous closure> (package:ferry_exec/src/typed_link.dart:131:46)
#3      _TypedLinkChain.request (package:ferry_exec/src/typed_link.dart:132:9)
#4      Client.request (package:ferry/ferry.dart:80:18)

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