Keep the Checkstyle configuration out of the jar

src/main/resources is packaged, so the Checkstyle configuration and the
copyright header shipped to everyone who depends on httpbin.  Neither is
read at runtime, unlike the logback.xml moved in the previous commit, but
neither belongs in the artifact either.

Move both to src/main/config, as S3Proxy has them, leaving
src/main/resources holding only what the endpoints actually serve.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M5JVWJN7L3NWmcsYvjeCfc

Author: gaul

pom.xml

@@ -142,8 +142,8 @@
           </execution>
         </executions>
         <configuration>
-          <configLocation>src/main/resources/checkstyle.xml</configLocation>
-          <headerLocation>src/main/resources/copyright_header.txt</headerLocation>
+          <configLocation>src/main/config/checkstyle.xml</configLocation>
+          <headerLocation>src/main/config/copyright_header.txt</headerLocation>
           <includeTestSourceDirectory>true</includeTestSourceDirectory>
           <violationSeverity>warning</violationSeverity>
         </configuration>