[c#] add support for init-only setters on generated classes

[enioluwas]

Closes #1154 by introducing an --init-only-properties switch for the Bond C# compiler.

Mostly just mimicked the implementation for the --readonly-properties switch. This was tricky to add unit tests for, because the init keyword was introduced in C# 9 / .NET 5. Please review and see if it's okay as-is. I considered adding a new unit test project for .NET 5+, but it wouldn't be compatible with VS 2017.

[chwarr]

Overall, LGTM. Can you add a changelog entry? This is a minor version bump for C#.

I forgot that init properties were a C# 9+ and .NET 5+ feature. Sigh. As you've noted, the CI build only uses the .NET Core 3.1 SDK today.

Bond still needs to support .NET Framework, so we can't require .NET 5 or newer. In issue #1143, Deprecating Bond support for EOL platforms and toolchains, @bhardwajs proposed a new minimum set of supported platforms, including updating the Windows CI build to Visual Studio 2019/dotnet. That needs to land before this can be merged. I don't see a way to test this new code without using a newer toolchain to build...

I'm not comfortable merging in code that doesn't have CI test coverage.

I do not have any time in the foreseeable future myself to work on Bond's CI builds. I'm going to need to rely on you or @bhardwajs to contribute the needed build modernization here.

Once that's done, then all the init code can be put behind conditional compilation and only be done on .NET 5+.

[enioluwas]

Alright, will hold off on this, and consider helping with #1143

[chwarr]

It has been more than six months since there have activity on this PR. I am going to close it for now. If you are still interested in working on this, let me know, and I can re-open this. The .NET build changes will still need to come first. Thanks.