iOS and tvOS support for OAuth browser-based clients, make initializers non-throwing

Two enhancements from @0xced - thanks!

Updated for Swift 5.1

On Linux, important networking functions are now in FoundationNetworking. This updates released code to conditionally include FoundationNetworking for Linux.

Swift 5 and SwiftNIO 2.0 Updates

The local webserver used by BrowserTokenProvider is now directly based on SwiftNIO and updated for SwiftNIO 2.0 and Swift 5.

Replace swift-server/HTTP with Vapor/HTTP

The BrowserTokenProviders now use Vapor/HTTP instead of the deprecated swift-server/HTTP. This should fix build problems with recent versions of Swift.

Support for Google Application Default Credentials

DefaultTokenProvider now provides full support for Google Application Default Credentials.