config service: avoid raising a json exception on non json input

The config service accepts in strings instead of json strings, however
this causes the normal control flow to throw a json parsing exception.
Since this happens so frequently it is using a measurable amount of CPU
time, so avoid it in the common case by testing if the string is json
first

http-service/src/test/java/net/runelite/http/service/config/ConfigServiceTest.java

@@ -26,6 +26,7 @@ package net.runelite.http.service.config;
 
 import com.google.common.collect.ImmutableMap;
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 import org.junit.Test;
 
@@ -53,4 +54,19 @@ public class ConfigServiceTest
 		assertTrue(ConfigService.validateJson("{\"key\": \"value\"}"));
 		assertTrue(ConfigService.validateJson("\n"));
 	}
+
+	@Test
+	public void testMaybeJson()
+	{
+		assertFalse(ConfigService.isMaybeJson("string"));
+		assertFalse(ConfigService.isMaybeJson("string with spaces"));
+
+		assertTrue(ConfigService.isMaybeJson("true"));
+		assertTrue(ConfigService.isMaybeJson("false"));
+		assertTrue(ConfigService.isMaybeJson("1"));
+		assertTrue(ConfigService.isMaybeJson("1.2"));
+		assertTrue(ConfigService.isMaybeJson("\"quote\""));
+		assertTrue(ConfigService.isMaybeJson("{\"key\": \"value\"}"));
+		assertTrue(ConfigService.isMaybeJson("[42]"));
+	}
 }