MDL-50783 Ajax: Configure how to call a webservice through db/service.php

MDL-50783 Ajax: Configure how to call a webservice through db/service.php

Now the db/service.php array can contain these extra keys to provide information
on how a webservice may be called:

    'ajax' => true (Default is false)

Replaces the xx_is_allowed_from_ajax callback.

    'loginrequired' => false (Default is true)

Means that this webservice can be called through lib/ajax/service-nosession.php
which sets NO_MOODLE_COOKIES to true (faster). This is only safe for webservices returning
static public data (e.g. get_string).