OfficeDevPnP.Core.AuthenticationManager is part of the PnP Core Library available here: https://www.nuget.org/packages/SharePointPnPCoreOnline
var authManager = new AuthenticationManager();
var ctx = authManager.GetWebLoginClientContext("https://contoso.sharepoint.com/");
Web web = ctx.Web;
User user = web.CurrentUser;
ctx.Load(web);
ctx.Load(user);
ctx.ExecuteQuery();
var authManager = new AuthenticationManager();
var ctx = authManager.GetWebLoginClientContext("https://contoso.sharepoint.com/");
Web web = ctx.Web;
User user = web.CurrentUser;
ctx.Load(web);
ctx.Load(user);
ctx.ExecuteQuery();