[dart-dio-next] improve api client

Created by: ahmednfwela

Improves api client generation

This PR generalizes many things that were hardcoded in the dart-dio-next generator, and introduces some improvements and bug fixes

1. [General] Added isContainer to CodegenResponse and called the missing setUniqueItems
2. [General] Changed debugModels, debugOperations behavior to debug the files AFTER post processing, making sure the developer sees the actual hash that is being fed to mustache files.
3. mapped file and binary to MultipartFile instead of Uint8List and let dio handle the rest
4. use inlineEnums when possible instead of just the underlying type
5. improved & generalized serializing response types
6. introduce connectTimeout and receiveTimeout as parameters to the api client file
7. split repetitive generation into separate mustache files

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

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