Add configuration.keys() returning the current keys

avaje-config/src/main/java/io/avaje/config/Configuration.java

@@ -491,6 +491,11 @@ default boolean enabled(String key, boolean enabledDefault) {
    */
   void reloadSources();
 
+  /**
+   * Return the property keys in this configuration.
+   */
+  Set<String> keys();
+
   /**
    * Return the number of configuration properties.
    */

avaje-config/src/main/java/io/avaje/config/CoreConfiguration.java

@@ -204,6 +204,11 @@ public Configuration forPath(String pathPrefix) {
     return new CoreConfiguration(this, newEntryMap, dotPrefix);
   }
 
+  @Override
+  public Set<String> keys() {
+    return properties.entries.keys();
+  }
+
   @Override
   public ListValue list() {
     return listValue;

avaje-config/src/main/java/io/avaje/config/CoreEntry.java

@@ -162,6 +162,10 @@ private boolean putIfChanged(String key, String value, String source) {
       return false;
     }
 
+    Set<String> keys() {
+      return entryMap.keySet();
+    }
+
     boolean containsKey(String key) {
       return entryMap.containsKey(key);
     }

avaje-config/src/test/java/io/avaje/config/CoreConfigurationTest.java

@@ -80,6 +80,7 @@ void forPath() {
     CoreConfiguration base = createSample();
     Configuration foo = base.forPath("foo");
 
+    assertThat(foo.keys()).contains("bar", "t", "f");
     assertThat(foo.size()).isEqualTo(3);
     assertThat(foo.getInt("bar")).isEqualTo(42);
     assertThat(foo.getBool("t")).isTrue();