Fix appender-ref resolution in complex appenders inside SiftingAppender

[vbolshutkin]

In my project we had a custom appender that implements AppenderAttachable interface which worked inside a SiftingAppender prior to Logback 1.3.

<appender name="FILE" class="ch.qos.logback.core.FileAppender">
  <file>target/test.log</file>
  ...
</appender>

  
<appender name="SIFT" class="ch.qos.logback.classic.sift.SiftingAppender">
  <discriminator>
    <key>MDC_KEY</key>
    <defaultValue>unknown</defaultValue>
  </discriminator>
  <sift>
    <appender name="CUSTOM-${MDC_KEY}" class="custom.package.CustomAppender">
      <appender-ref ref="FILE" />
    </appender>
  </sift>
</appender>	

In Logback 1.5.18 it's only working in root scope as

    <appender name="CUSTOM" class="custom.package.CustomAppender">
      <appender-ref ref="FILE" />
    </appender>

But when put inside SiftingAppender, the appender-ref behaves as an ImplicitModel and is ignored.

This PR fixes the issue by allowing appender-ref inside appender/sift/appender/appender-ref . Direct usage of appender-ref in sift is still prohibited (as it has no sense)