Add new Lazy<T>(T value) ctor to System.Runtime contract and add tests

[justinvp]

Now that the implementation of #14947 has been merged for CoreCLR (dotnet/coreclr#6304) and CoreRT (dotnet/corert#1538), the new API needs to be exposed in the System.Runtime contract (and versioned as appropriate) and tests added.

[justinvp]

Do we need a separate issue tracking adding the new API to the full framework? Or would that be tracked internally by Microsoft? What about Mono/Xamarin?

[weshaggard]

@justinvp @terrajobst @danmosemsft @jkotas I just created a new api-needs-exposed label to mark corefx issues for new APIs that are implemented but not yet exposed.