[SPARK-22710] ConfigBuilder.fallbackConf should trigger onCreate function

## What changes were proposed in this pull request?
I was looking at the config code today and found that configs defined using ConfigBuilder.fallbackConf didn't trigger onCreate function. This patch fixes it.

This doesn't require backporting since we currently have no configs that use it.

## How was this patch tested?
Added a test case for all the config final creator functions in ConfigEntrySuite.

Author: Reynold Xin <[email protected]>

Closes #19905 from rxin/SPARK-22710.

Author: rxin

core/src/main/scala/org/apache/spark/internal/config/ConfigBuilder.scala

@@ -235,7 +235,9 @@ private[spark] case class ConfigBuilder(key: String) {
   }
 
   def fallbackConf[T](fallback: ConfigEntry[T]): ConfigEntry[T] = {
-    new FallbackConfigEntry(key, _alternatives, _doc, _public, fallback)
+    val entry = new FallbackConfigEntry(key, _alternatives, _doc, _public, fallback)
+    _onCreate.foreach(_(entry))
+    entry
   }
 
   def regexConf: TypedConfigBuilder[Regex] = {

core/src/test/scala/org/apache/spark/internal/config/ConfigEntrySuite.scala

@@ -288,4 +288,24 @@ class ConfigEntrySuite extends SparkFunSuite {
     conf.remove(testKey("b"))
     assert(conf.get(iConf) === 3)
   }
+
+  test("onCreate") {
+    var onCreateCalled = false
+    ConfigBuilder(testKey("oc1")).onCreate(_ => onCreateCalled = true).intConf.createWithDefault(1)
+    assert(onCreateCalled)
+
+    onCreateCalled = false
+    ConfigBuilder(testKey("oc2")).onCreate(_ => onCreateCalled = true).intConf.createOptional
+    assert(onCreateCalled)
+
+    onCreateCalled = false
+    ConfigBuilder(testKey("oc3")).onCreate(_ => onCreateCalled = true).intConf
+      .createWithDefaultString("1.0")
+    assert(onCreateCalled)
+
+    val fallback = ConfigBuilder(testKey("oc4")).intConf.createWithDefault(1)
+    onCreateCalled = false
+    ConfigBuilder(testKey("oc5")).onCreate(_ => onCreateCalled = true).fallbackConf(fallback)
+    assert(onCreateCalled)
+  }
 }