[dart-dio-next] improve api client (!10188) · Merge requests · OpenAPI Tools / openapi-generator

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

[General] Added isContainer to CodegenResponse and called the missing setUniqueItems
[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.
mapped file and binary to MultipartFile instead of Uint8List and let dio handle the rest
use inlineEnums when possible instead of just the underlying type
improved & generalized serializing response types
introduce connectTimeout and receiveTimeout as parameters to the api client file
split repetitive generation into separate mustache files

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.
Run the following to build the project and update samples:
```
./mvnw clean package 
./bin/generate-samples.sh
./bin/utils/export_docs_generators.sh
```
Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
File the PR against the correct branch: master (5.3.0), 6.0.x
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