MULE-7268: Add support for GZIP compression when proxying a web service

There is no support for handling compression when proxying a web service built in with the proxy client and proxy service. Doing it in the interceptors instead that using Mule's transformers is better for the streaming support.
The solution is to implement a GZIPFeature that injects the GZIP In and Out Interceptors on the proxy client or service. Since injecting it in the proxy client and service builders to work out of the box might break backwards compatibility, it requires it to be configured manually:

   <cxf:proxy-client>
      <cxf:features>
          <spring:bean class="org.mule.module.cxf.feature.ProxyGZIPFeature" />
      </cxf:features> 
   </cxf:proxy-client>

The interceptors should handle the compression and decompression based on the Content-Encoding, which needs to be "gzip" or "x-gzip". The property is not deleted after is decompressed, since we are proxying and most likely copying all of the properties. For compressing the property needs to be already specified.