[C#][aspnet] use default in model constructors, supports enums

[jimschubert]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

This addresses a couple of issues. As originally reported in #3608 and added to the C# client generator in #4145, this updates model constructors to use default() rather than attempting to initialize parameters to nulls. I didn't modify the NancyFX generator here, because it relies on object initialization and doesn't perform the same types of constructor validations or DataAnnotations/JSON validation.

The petstore.yaml has inner enums, so to get the sample code working against this yaml I've also added support for enums and inner enums, and regenerated the sample.

Please note if you're having issues with newer versions of dotnet CLI, there's a known issue (https://github.com/dotnet/corefx/issues/9171) with OS X/macOS support which requires linking openssl manually:

$ ln -s /usr/local/opt/openssl/lib/libcrypto.1.0.0.dylib /usr/local/lib/
$ ln -s /usr/local/opt/openssl/lib/libssl.1.0.0.dylib /usr/local/lib/

To test:

cd samples/server/petstore/aspnetcore
sh build.sh

NOTE:

If you receive the following error on build, you'll need to perform the openssl symlinks mentioned above:

Unhandled Exception: System.TypeInitializationException: The type initializer for 'Crypto' threw an exception. ---> System.TypeInitializationException: The type initializer for 'CryptoInitializer' threw an exception. ---> System.DllNotFoundException: Unable to load DLL 'System.Security.Cryptography.Native': The specified module could not be found.
 (Exception from HRESULT: 0x8007007E)
   at Interop.CryptoInitializer.EnsureOpenSslInitialized()
   at Interop.CryptoInitializer..cctor()
   --- End of inner exception stack trace ---
   at Interop.Crypto..cctor()
   --- End of inner exception stack trace ---
   at Interop.Crypto.GetRandomBytes(Byte* buf, Int32 num)
   at System.IO.Path.GetCryptoRandomBytes(Byte* bytes, Int32 byteCount)
   at System.IO.Path.GetRandomFileName()
   at Microsoft.DotNet.InternalAbstractions.TemporaryDirectory..ctor()
   at Microsoft.Extensions.EnvironmentAbstractions.DirectoryWrapper.CreateTemporaryDirectory()
   at Microsoft.DotNet.Configurer.NuGetPackagesArchiver..ctor()
   at Microsoft.DotNet.Cli.Program.ConfigureDotNetForFirstTimeUse(INuGetCacheSentinel nugetCacheSentinel)
   at Microsoft.DotNet.Cli.Program.ProcessArgs(String[] args, ITelemetry telemetryClient)
   at Microsoft.DotNet.Cli.Program.Main(String[] args)
build.sh: line 8: 45627 Abort trap: 6           dotnet restore src/IO.Swagger/

[wing328]

@jimschubert thanks for the PR and the solution to address the SSL issue.