Support ASP.NET 5 using RC1

[martincostello]

This PR adds support for ASP.NET 5, starting with preview 6 through to RC1.

[martincostello]

Once RC1 lands (after preview 8), I'll look at merging this to a rel/5.0.0 (or similar) so that we start publishing 5.0 packages to MyGet.

[martincostello]

For RC1 we can use the new "one-shot" hash methods (dotnet/aspnetcore#24696) in these two providers:

AspNet.Security.OAuth.Providers/src/AspNet.Security.OAuth.Deezer/DeezerAuthenticationHandler.cs

Line 131 in 28f6c0d

using var sha256 = HashAlgorithm.Create("SHA256");

AspNet.Security.OAuth.Providers/src/AspNet.Security.OAuth.Odnoklassniki/OdnoklassnikiAuthenticationHandler.cs

Line 42 in 28f6c0d

using var algorithm = HashAlgorithm.Create("MD5");

We can also leverage the new Convert.ToHexString() method here:

AspNet.Security.OAuth.Providers/src/AspNet.Security.OAuth.Odnoklassniki/OdnoklassnikiAuthenticationHandler.cs

Lines 80 to 90 in 28f6c0d

	private static string GetHash(HashAlgorithm algorithm, string input)
	{
	var builder = new StringBuilder();
	foreach (byte b in algorithm.ComputeHash(Encoding.UTF8.GetBytes(input)))
	{
	builder.Append(b.ToString("x2", CultureInfo.InvariantCulture));
	}
	return builder.ToString();
	}

[martincostello]

I think something in Arcade needs updating somewhere, as now the build gets this error with preview 8:

/home/runner/work/AspNet.Security.OAuth.Providers/AspNet.Security.OAuth.Providers/.dotnet/sdk/5.0.100-preview.8.20417.9/NuGet.targets(128,5): error : Invalid restore input. Invalid target framework 'unsupported'. Input files: /home/runner/work/AspNet.Security.OAuth.Providers/AspNet.Security.OAuth.Providers/.packages/microsoft.dotnet.arcade.sdk/5.0.0-beta.20180.5/tools/Tools.proj. [/home/runner/work/AspNet.Security.OAuth.Providers/AspNet.Security.OAuth.Providers/.packages/microsoft.dotnet.arcade.sdk/5.0.0-beta.20180.5/tools/Tools.proj]

Vanilla dotnet build and Visual Studio builds are fine.

[martincostello]

See #472.