[dart-dio] [dart] Unify both dart generators into just dart

[ahmednfwela]

based on: #14291
Blocked on:

• [dart-next] Refactor to decouple serialization from networking #15485
• [REQ] [dart-dio] [internal refactor] Unify test schemas #15449

this PR adds support for using http library instead of dio

it's still WIP and highly experimental, suggestions welcome

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
  

  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.
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[wing328]

What about using library (global option, not generator option) instead?

[ahmednfwela]

currently the serializationLibrary is assigned to library global option (this was introduced in #9980)
I was going to make another PR that targets v7.0.0, with the correct parameters assigned, but I am not sure if this is the right approach

[ahmednfwela]

relevant line: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/DartDioClientCodegen.java#L162

[ahmednfwela]

Replaced by #15485