Store multiple bodyParams with mediaType appropriately

[jharriman]

PR checklist

New to the project, so while I did my best to answer these questions, I know I did not fully grok the release process and some of the larger concerns that may need to be addressed by my change.

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.

Not updating any language templates, so I'm guessing I don't need to do this?

• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.

This doesn't seem to be a breaking change (new to this project though, so I could be missing something), so master?

• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

PR targets core, not a specific programming language.

Description of the PR

Addresses the issue raised in #3990:

• Converts DefaultCodgen.fromRequestBody to return a List<CodegenParameter> instead of a single CodegenParameter (since requestBody's often contain multiple Content Types.
• Adds a test for creating multiple bodyParams when there are multiple Content Types.
• Updates callers that depend on the singleton functionality to operate as thought there is a singleton (tried to keep the change as simple as possible for now, but these call sites should properly handle receiving multiple CodegenParameters.

[spacether]

Thank you for your PR. We do not need it because this information was added in #10973