Add shortcut functions for applying headers

[veewee]

Q	A
Type	improvement
BC Break	no
Fixed issues	php-soap/psr18-transport#3

Summary

This PR provides some builders for dealing with soap headers:

$builder = new SoapHeaders(
    new SoapHeader(
        $targetNamespace,
        'x:Auth',
        children(
            namespaced_element($targetNamespace, 'x:user', value('josbos')),
            namespaced_element($targetNamespace, 'x:password', value('topsecret'))
        ),
        // Optionally, you can provide additional configurators for setting
        // SOAP-ENV specific attributes:
        Actor::next(),
        new MustUnderstand()
    ),
    $header2,
    $header3
);

$headers = $doc->build($builder);
$doc->manipulate(new PrependSoapHeaders(...$headers));