Merge ServiceLoader registrations when shading xtable-utilities (#906)

The shade plugin had no ServicesResourceTransformer, so every
META-INF/services/* file was last-one-wins: whichever artifact shade
processed last silently overwrote the others.

Two visible consequences. The bundled jar could not read Delta sources
at all, because Delta's DataSourceRegister entry lost to Spark's and
sourceFormat: DELTA died with ClassNotFoundException: delta.DefaultSource
despite the class being present. And only one Hadoop FileSystem
registration survived, which is why xtable-hadoop-defaults.xml has to
name fs.file.impl, fs.s3a.impl and the GCS classes by hand.

Closes #905.

Author: slachiewicz

xtable-utilities/pom.xml

@@ -286,6 +286,7 @@
                             <resource>META-INF/NOTICE</resource>
                             <file>target/classes/META-INF/NOTICE-bundled</file>
                         </transformer>
+                        <transformer implementation="org.apache.maven.plugins.shade.resource.ServicesResourceTransformer"/>
                         <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                             <mainClass>org.apache.xtable.utilities.RunSync</mainClass>
                         </transformer>