[JAX-RS] Fix parameter names using {{baseName}}

[ePaul]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change.
  → bin/jaxrs-*.sh
• [?] Filed the PR against the correct branch: master for non-breaking changes. (???)

Description of the PR

This is a part of a series of fixes for #4898.
It fixes the wrong name translations for all kinds of form parameters in `jaxrs` and all its library variants, when the original parameter name doesn't fit the naming conventions (e.g. camelCase) or conflicts with a reserved word.

For plain JaxRS, only file upload form parameters and form parameters with a `x-multipart` vendor extension were affected. For most library variants, all kinds of form parameters were affected.

Generating the samples again shows quite some differences (mainly for reserved word conflicts). I didn't really got to test the changes yet, but the new ones look better to my eye. I'm not sure if the petstore samples cover every case this changes, though.

While this is a bug-fix, I'm not sure whether it qualifies as *non-breaking change*. One could say that those clients/servers previously didn't adhere to their swagger definition, and now they do. But if we use a broken client and a broken server together, it would still have "worked", and won't work when upgrading just one of them.

This needs review (and potentially tests) by Java/JaxRS experts. /cc @cbornet @xhh @kolyjjj 

[ePaul]

Rebased onto master after #4901 was merged.

[wing328]

The change looks good to me 👍