http service: use gson instead of jackson for json serialization

This fixes deserailization of the loot timestamps for the loot tracker,
and maybe other things. Note this does not use the same Gson instance as
http-api as Spring creates its own Gson, however they are both default
Gson instances.

http-service/src/main/java/net/runelite/http/service/SpringWebMvcConfigurer.java

@@ -24,22 +24,35 @@
  */
 package net.runelite.http.service;
 
+import java.util.List;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.http.MediaType;
+import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.converter.json.GsonHttpMessageConverter;
 import org.springframework.web.servlet.config.annotation.ContentNegotiationConfigurer;
 import org.springframework.web.servlet.config.annotation.EnableWebMvc;
 import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;
 
-/**
- * Configure .js as application/json to trick Cloudflare into caching json responses
- */
 @Configuration
 @EnableWebMvc
-public class SpringContentNegotiationConfigurer extends WebMvcConfigurerAdapter
+public class SpringWebMvcConfigurer extends WebMvcConfigurerAdapter
 {
+	/**
+	 * Configure .js as application/json to trick Cloudflare into caching json responses
+	 */
 	@Override
 	public void configureContentNegotiation(ContentNegotiationConfigurer configurer)
 	{
 		configurer.mediaType("js", MediaType.APPLICATION_JSON);
 	}
+
+	/**
+	 * Use GSON instead of Jackson for JSON serialization
+	 * @param converters
+	 */
+	@Override
+	public void configureMessageConverters(List<HttpMessageConverter<?>> converters)
+	{
+		converters.add(new GsonHttpMessageConverter());
+	}
 }