Support setting properties on HttpClientHandler

[maartenba]

Support setting properties on HttpClientHandler in both BearerTokenConnection and UsernamePasswordConnection. Scenarios could be configuring SSL etc.

An example could be setting the HttpClientHandler's ServerCertificateCustomValidationCallback:

var connection = new BearerTokenConnection(
    "https://ytsharp.myjetbrains.com/youtrack/", 
    "perm:abcdefghijklmn", 
    handler =>
    {
        handler.ServerCertificateCustomValidationCallback =
            (sender, certificate, chain, sslPolicyErrors) => true;
    });

(Note the above is not a secure example as it disables SSL certificate checks!)