[CSharp] add optional flag "optionalEmitDefaultValues" and update template

[sdoeringNew]

PR checklist

• Read the contribution guidelines. But I'm unsure if I named the vendorExtension flag correctly. I do not put the csharp prefix as the other extensions in CSharp did not have this prefix, too.
• Ran the shell script under ./bin/ to update Petstore sample for CSharp... But I haven't committed them because there were many other changes from previous template changes - so nobody seem to commit them. At least I saw that the flag worked as expected.
• Filed the PR against the correct branch: master
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.
• Waited for the checks to have passed.

Description of the PR

This commit addresses issue #3274 and adds the proposed optionalEmitDefaultValues flag.

Invite cool reviewers

@mandrean (2017/08), @jimschubert (2017/09)

[wing328]

@sdoeringNew thanks for the PR. We can go with this approach to see if users have feedback on this.

[wing328]

I would recommend creating a new branch as per git best practic for your upcoming PRs.

[wing328]

@sdoeringNew thanks for the PR, which has been included in v4.2.0 release: https://twitter.com/oas_generator/status/1189824932345069569