[WindowsPlatform] Add a way to launch a WinForm modally.
[Core] Lift proxy helpers from NuGet into Core.
[WindowsPlatform] Start implementing proxy support.
[Core] Register the extension node name correctly.
[WindowsPlatform] Fix implementation of proxy credential provider
[Core] Add API for storing u/p pairs to PasswordService
[Core] Save proxy credentials via the password service

Also ensure that we load them out again when we hit our
CredentialStore.GetCredentials implementation.
[WindowsPlatform] Make native class static.
[WindowsPlatform] Add fallback API for Windows XP.
[MacPlatform] Migrate MacKeychainPasswordProvider and add new API

Implement the new API for AddWebUserNameAndPassword and
GetWebUserNameAndPassword for IPasswordService changes, and
implement the Keychain API for looking this up.

Also simplify RequestHelper a bit.
[WindowsPlatform] Correctly state the type of credential we need
[WindowsPlatform] Implement a password storage provider for Windows
[Core] Fix the credential store to store both request and proxy credentials

Up until now, it would not store proxy credentials at all.
[WindowsPlatform] Null check the URI in WindowsProxyCredentialProvider
[WindowsPlatform] Fix the password provider to read/write properly

Before, we would either not write anything, come up with read errors,
or read garbage. Fix everything by following an MSDN blog example,
learning a bit about marshalling, and marshalling the struct
correctly.
[Core] Don't cache proxy credentials if no proxy is set.
[Core] Wrap the response properly.
[Core] Support passing current credentials to provider

The provider can prefill their window to present these to the user.
[WindowsPlatform] Prefill credential prompt with old credentials
[MacPlatform] Fix keychain attribute interop for FindInternetUserNameAndPassword.