[dart2] Include request body on DELETE call

Created by: yholkamp

This change updates the generated ApiClient to include the request body on DELETE calls. Per the RFC on page 2, DELETE calls are allowed to have a request body, even though the behavior may be undefined. I've not come across many APIs that use this behavior but there are a few, in particular the Ory Kratos API does so, not forwarding the body prevents such an API from being used. An API method that does not define a request body should continue to function as expected as the body would be null in that scenario.

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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