Switch to an overridable onFailure method in InngestFunction

This removes the `WithFailureHandler` interface and instead provides an
overridable `onFailure` method in `InngestFunction` that can be used to
define a function to be called when the function fails.

It's used in the same way but without the need to implement an interface

```kotlin

class MyFunction : InngestFunction() {
    override fun execute(ctx: FunctionContext, step: Step): Any? {
        // Do something
    }

    override fun onFailure(ctx: FunctionContext, step: Step): Any? {
        // Handle failure
    }
}

```

inngest-spring-boot-demo/src/main/java/com/inngest/springbootdemo/testfunctions/WithOnFailureFunction.java

@@ -4,7 +4,7 @@
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
-public class WithOnFailureFunction extends InngestFunction implements WithFailureHandler {
+public class WithOnFailureFunction extends InngestFunction {
     @NotNull
     @Override
     public InngestFunctionConfigBuilder config(InngestFunctionConfigBuilder builder) {

inngest-spring-boot-demo/src/test/java/com/inngest/springbootdemo/WithOnFailureIntegrationTest.java

@@ -19,11 +19,6 @@
 @IntegrationTest
 @Execution(ExecutionMode.CONCURRENT)
 class WithOnFailureIntegrationTest {
-    @BeforeAll
-    static void setup(@Autowired CommHandler handler) {
-        handler.register("http://localhost:8080");
-    }
-
     @Autowired
     private DevServerComponent devServer;
 
@@ -35,7 +30,7 @@ static void setup(@Autowired CommHandler handler) {
     @Test
     void testWithOnFailureShouldCallOnFailure() throws Exception {
         String eventName = "test/with-on-failure";
-        String eventId = InngestFunctionTestHelpers.sendEvent(client, eventName).first();
+        String eventId = InngestFunctionTestHelpers.sendEvent(client, eventName).getIds()[0];
 
         Thread.sleep(sleepTime);
 

inngest-test-server/src/main/kotlin/com/inngest/testserver/SlackFailureReport.kt

@@ -2,9 +2,7 @@ package com.inngest.testserver
 
 import com.inngest.*
 
-class SlackFailureReport :
-    InngestFunction(),
-    WithFailureHandler {
+class SlackFailureReport : InngestFunction() {
     override fun config(builder: InngestFunctionConfigBuilder): InngestFunctionConfigBuilder =
         builder
             .id("always-fail-fn")

inngest/src/main/kotlin/com/inngest/InngestFunction.kt

@@ -30,7 +30,9 @@ abstract class InngestFunction {
     }
 
     internal fun toFailureHandler(appId: String): InternalInngestFunction? {
-        if (this is WithFailureHandler) {
+        val onFailureMethod = this.javaClass.getMethod("onFailure", FunctionContext::class.java, Step::class.java)
+
+        if (onFailureMethod.declaringClass != InngestFunction::class.java) {
             val fnConfig = buildConfig()
             val fullyQualifiedId = "$appId-${fnConfig.id}"
             val fnName = fnConfig.name ?: fnConfig.id
@@ -45,4 +47,20 @@ abstract class InngestFunction {
         }
         return null
     }
+
+    /**
+     * Provide a function to be called if your function fails, meaning
+     * that it ran out of retries and was unable to complete successfully.
+     *
+     * This is useful for sending warning notifications or cleaning up
+     * after a failure and supports all the same functionality as a
+     * regular handler.
+     *
+     * @param ctx The function context including event(s) that triggered the function
+     * @param step A class with methods to define steps within the function
+     */
+    open fun onFailure(
+        ctx: FunctionContext,
+        step: Step,
+    ): Any? = null
 }