Apply style and explode values from encoding for form-encoded request body parameters

[harmony7]

This PR adds support for the style and explode values on the encoding object onto request body parameters that are submitted as application/x-www-form-urlencoded or multipart.

The Encoding Object allows the specification of style and explode similar to how they can be applied to operation parameters.

There has already existed code that would apply contentType from this Encoding Object, but this was the only value that was being applied. This PR renames that function from setParameterContentType to setParameterEncodingValues, and now also applies style and explode as appropriate.

I've added tests to DefaultCodegenTest.testRequestBodyContent() against some new definitions in resources/3_0/content-data.yaml to show that these work.

This should fix issues such as #3781 and #11256.

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[harmony7]

@wing328 Are you able to look at this for me when you have a minute, or give it to the right person? This would help us. I'd like to avoid maintaining my own fork if possible. Thanks!

[rulrok]

Looking forward for this feature!

[wing328]

Thanks for the PR. I'll try to review over the weekend.

cc @OpenAPITools/generator-core-team

Have a nice weekend.

[wing328]

LGTM but I've not tested it locally.

[wing328]

Katsuyuki-san, thanks for the PR, which has been merged into master.

When you've time tomorrow or next week, can you please PM me via Slack? https://join.slack.com/t/openapi-generator/shared_invite/zt-12jxxd7p2-XUeQM~4pzsU9x~eGLQqX2g