Support /t in the project settings for sgen

[huanwu]

Support type parameter in project settings so the customer can serialize just single class.
Refer dotnet/corefx#24491

[AroglDarthu]

We've tried to use the generator in a project that has classes based on XSD schema's. The resulting dll is over 3 MB. Without using the /t parameter, the xml serializer generator just hangs itself. Never ends. No failures, no exception. Just hangs.
What we did for now is to include the generator dll directly in our project and we created a target that passes --type on to it. Works like a charm, but it is a very cumbersome method...

[jiayi11]

@huanwu Do we want to support customers to specify the types in their project file PropertyGroup like the following?

<PropertyGroup>
    <SerializationType>project1.type1; project1.type2</SerializationType>
</PropertyGroup>

[jiayi11]

Pr dotnet/corefx#36085 merged. Closing this issue.
Users will be able to modify the sgen command parameters using msbuild properties in their csproj file like the following:

<PropertyGroup>
    <SGenReferences>C:\myfolder\abc.dll;C:\myfolder\def.dll</SGenReferences>
    <SGenTypes>SgenTestProgram.MyType1;SgenTestProgram.MyType2</SGenTypes>
    <SGenProxyTypes>false</SGenProxyTypes>
    <SGenVerbose>true</SGenVerbose>
    <SGenKeyFile>mykey.snk</SGenKeyFile>
    <SGenDelaySign>true</SGenDelaySign>
</PropertyGroup>