[Feature Request] XSS/EscapeOutput: add ability to pass (auto)escaped Methods

PHPCS 3.1, WPCS 0.13.0, PHP 5.6.30

A backslash used to indicate the global namespace prevents customAutoEscapedFunctions from being recognized.

I have a ruleset like this:

<?xml version="1.0"?>
<ruleset name="WordPress-Me">
  <rule ref="WordPress.XSS.EscapeOutput">
    <properties>
      <property name="customAutoEscapedFunctions" value="ESPN_AMP, AMP_HTML_Utils" type="array" />
    </properties>
  </rule>
</ruleset>

And some code like this:

public static function amp_additional_css_styles() {
  $css = 'color: black;';
  echo \ESPN_AMP::sanitize_meta_css( $css );
}

which generates this error:

 373 | ERROR | Expected next thing to be an escaping function (see Codex for 'Data Validation'), not '\'
     |       | (WordPress.XSS.EscapeOutput.OutputNotEscaped)

If I drop the backslash (\):

public static function amp_additional_css_styles() {
  $css = 'color: black;';
  echo ESPN_AMP::sanitize_meta_css( $css );
}

Then PHPCS no longer finds the error.

Start at the WordPress.XSS.EscapeOutput rule and reproduce the issue with the shown customAutoEscapedFunctions ruleset and namespaced method call. Trace how the sniff recognizes configured escaping methods, then verify that the fully qualified call with a leading backslash is accepted while existing unescaped calls remain reported.