[dart][dart-dio] API class/template refactoring (!8825) · Merge requests · OpenAPI Tools / openapi-generator

first construct a RequestOptions instance with all parameters etc. inline
then construct and serialize the body data
then send the request and await the response
then deserialize the response
always use the specifiedType for built_value (de)serialization instead of a mix of ways (this does the same as finding the serializer directly)
use indent of 2 spaces (as per Dart convention)
prefix all local variables to prevent name clashes with parameters
add 2 tests that (de)serialize collections and add API key header

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, 5.1.x, 6.0.x
If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

CC @swipesight (2018/09) @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12)