Add metadata support for types in arbitrary modules

Previously, if a ConfigurationProperties had a nested type or was
extending from a type located outside the compilation unit, no
metadata discovered on the source code was available (documentation and
explicit default value, if any). This typically happens when such a type
resides in another module.

This commit introduces `@ConfigurationPropertiesSource` as a way to
annotate such type and have metadata generated for them in their own
module.

Type-metadata is generated as one file per type and is reused
transparently whenever that type is used. As for module metadata, an
additional file can be crafted manually and will be merged when the
metadata for the type is generated.

The following is an example structure with two types where one has
an additional metadata:

META-iNF/
  spring/
    configuration-properties/
      additional/
        com.example.SourceOne.json
      com.example.SourceOne.json
      com.example.SourceTwo.json

Those files are used only by the annotation processor and are not meant
to be public API.

See gh-18366

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/appendix/configurationmetadata/annotationprocessor/automaticmetadatageneration/source/Host.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2012-2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.appendix.configurationmetadata.annotationprocessor.automaticmetadatageneration.source;
+
+import org.springframework.boot.context.properties.ConfigurationPropertiesSource;
+
+@ConfigurationPropertiesSource
+public class Host {
+
+	/**
+	 * IP address to listen to.
+	 */
+	private String ip = "127.0.0.1";
+
+	/**
+	 * Port to listener to.
+	 */
+	private int port = 9797;
+
+	// @fold:on // getters/setters ...
+	public String getIp() {
+		return this.ip;
+	}
+
+	public void setIp(String ip) {
+		this.ip = ip;
+	}
+
+	public int getPort() {
+		return this.port;
+	}
+
+	public void setPort(int port) {
+		this.port = port;
+	}
+	// @fold:off
+
+}

spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/configurationmetadata/annotationprocessor/automaticmetadatageneration/source/Host.kt

@@ -0,0 +1,19 @@
+package org.springframework.boot.docs.configurationmetadata.annotationprocessor.automaticmetadatageneration.source
+
+import org.springframework.boot.context.properties.ConfigurationPropertiesScan
+
+@ConfigurationPropertiesScan
+class Host {
+
+	/**
+	 * IP address to listen to.
+	 */
+	var ip: String = "127.0.0.1"
+
+	/**
+	 * Port to listener to.
+	 */
+	var port = 9797
+
+
+}