Makes health configuration optional (line#2104)

A previous change fixed health status mapping, but in the process made
it required to have health autoconfiguration. This fixes the below
error:

```
Parameter 3 of method actuatorServerConfigurator in com.linecorp.armeria.spring.actuate.ArmeriaSpringActuatorAutoConfiguration required a bean of type 'org.springframework.boot.actuate.health.HealthStatusHttpMapper' that could not be found.
```

spring/boot-actuator-autoconfigure/src/main/java/com/linecorp/armeria/spring/actuate/ArmeriaSpringActuatorAutoConfiguration.java

@@ -103,6 +103,12 @@ WebEndpointDiscoverer webEndpointDiscoverer(
                                          filters.orderedStream().collect(toImmutableList()));
     }
 
+    @Bean
+    @ConditionalOnMissingBean // In case HealthEndpointAutoConfiguration is excluded
+    HealthStatusHttpMapper healthStatusHttpMapper() {
+        return new HealthStatusHttpMapper();
+    }
+
     @Bean
     ArmeriaServerConfigurator actuatorServerConfigurator(
             WebEndpointsSupplier endpointsSupplier,