Support SpnEndpointIdentity

[iamjasonp]

The ctor currently throws PlatformNotSupportedException.

[hongdai]

See below table for what work in RTM.

Feature	[UWP]	[Windows]	[Linux OSs]	[Mac OS X]
SPNIdentity/TCP	🚫	✅	✅	✅
SPNIdentity/HTTP	🚫	🚫	🚫	🚫

[zhenlan]

This is blocked by dotnet/corefx#4502

[roncain]

Issue https://github.com/dotnet/corefx/issues/4502 is slated for 'future' so this probably will not be in 1.1