[BUG][csharp-netcore] Fix Multi Files for the same FormField

[joaocmendes]

Fix issue #11132

FileParameters at Request Options is a Dictionary so we weren't able to send more than one file with the same key.
Refactor FileParamerts to Multimap to enable sending more than one file with the same key.

I did the development only for csharp-netcore but it has no impacts and can be made to the others generators.

If you find it useful say it at the review.
@mandrean
@shibayan
@Blackclaws
@lucamazzanti

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[wing328]

C# tests (with updates samples passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/42144330 )

[joaocmendes]

I've done a force push because I follow the instructions to generate the samples and it added a lot of changes for slash replacement. I reset and commit only things related to this issue.
I don't know why the build failed, I tried to look to logs but didn't add much. May we re-run the job?

[wing328]

The bitrise CI failure can be ignored.

[wing328]

@joaocmendes thanks for the PR, which has been merged into master.