Add and integrate build-time config source build item

Part of quarkusio#3008

core/deployment/src/main/java/io/quarkus/deployment/builditem/BuildTimeConfigurationSourceBuildItem.java

@@ -0,0 +1,43 @@
+package io.quarkus.deployment.builditem;
+
+import java.util.function.Supplier;
+
+import org.eclipse.microprofile.config.spi.ConfigSource;
+import org.wildfly.common.Assert;
+
+import io.quarkus.builder.item.MultiBuildItem;
+
+/**
+ * Define an additional configuration source which is used at build time.
+ */
+public final class BuildTimeConfigurationSourceBuildItem extends MultiBuildItem {
+    private final Supplier<ConfigSource> configSourceSupplier;
+
+    /**
+     * Construct a new instance.
+     *
+     * @param configSourceSupplier the config source supplier (must not be {@code null})
+     */
+    public BuildTimeConfigurationSourceBuildItem(final Supplier<ConfigSource> configSourceSupplier) {
+        Assert.checkNotNullParam("configSourceSupplier", configSourceSupplier);
+        this.configSourceSupplier = configSourceSupplier;
+    }
+
+    /**
+     * Construct a new instance.
+     *
+     * @param configSource the config source (must not be {@code null})
+     */
+    public BuildTimeConfigurationSourceBuildItem(final ConfigSource configSource) {
+        this(() -> Assert.checkNotNullParam("configSource", configSource));
+    }
+
+    /**
+     * Get the config source supplier.
+     *
+     * @return the config source supplier
+     */
+    public Supplier<ConfigSource> getConfigSourceSupplier() {
+        return configSourceSupplier;
+    }
+}

core/deployment/src/main/java/io/quarkus/deployment/steps/ConfigurationSetup.java

@@ -20,6 +20,7 @@
 import java.util.function.Consumer;
 import java.util.function.UnaryOperator;
 
+import io.quarkus.deployment.builditem.BuildTimeConfigurationSourceBuildItem;
 import org.eclipse.microprofile.config.Config;
 import org.eclipse.microprofile.config.spi.ConfigBuilder;
 import org.eclipse.microprofile.config.spi.ConfigProviderResolver;
@@ -170,6 +171,7 @@ public ConfigurationSetup() {
      * @param resourceConsumer
      * @param niResourceConsumer
      * @param runTimeDefaultConsumer
+     * @param configSourceItems the build-time config source build items
      * @param extensionClassLoaderBuildItem the extension class loader build item
      * @param archiveRootBuildItem the application archive root
      * @throws IOException
@@ -183,6 +185,7 @@ public void initializeConfiguration(
             Consumer<GeneratedResourceBuildItem> resourceConsumer,
             Consumer<SubstrateResourceBuildItem> niResourceConsumer,
             Consumer<RunTimeConfigurationDefaultBuildItem> runTimeDefaultConsumer,
+            List<BuildTimeConfigurationSourceBuildItem> configSourceItems,
             ExtensionClassLoaderBuildItem extensionClassLoaderBuildItem,
             ArchiveRootBuildItem archiveRootBuildItem) throws IOException, ClassNotFoundException {
 
@@ -223,6 +226,13 @@ public void initializeConfiguration(
         final DefaultValuesConfigurationSource defaultSource = new DefaultValuesConfigurationSource(
                 buildTimeConfig.getLeafPatterns());
         builder.withSources(inJar, defaultSource);
+        final int cnt = configSourceItems.size();
+        final ConfigSource[] configSources = new ConfigSource[cnt];
+        int i = 0;
+        for (BuildTimeConfigurationSourceBuildItem item : configSourceItems) {
+            configSources[i++] = item.getConfigSourceSupplier().get();
+        }
+        builder.withSources(configSources);
 
         // populate builder with all converters loaded from ServiceLoader 
         ConverterSupport.populateConverters(builder);