fix: secretmanager to be an optional dep in autoconfig module (#3706)

* fix: secretmanager to be an optional dep in autoconfig module

Fixes #3560
It markes the secretmanager dependency as optional.
The config loader in the Secret Manager autoconfig was assuming the existence of the now optional Secret Manager integration
.
We now check for the class to be present before resolving the properties

* Update spring-cloud-gcp-autoconfigure/src/main/java/com/google/cloud/spring/autoconfigure/secretmanager/SecretManagerConfigDataLocationResolver.java

* improve comment

* fix comment checkstyle

Author: diegomarquezp

spring-cloud-gcp-autoconfigure/pom.xml

@@ -275,10 +275,7 @@
         <dependency>
             <groupId>com.google.cloud</groupId>
             <artifactId>spring-cloud-gcp-secretmanager</artifactId>
-            <!-- A few starters need this dependency to validate secrets -->
-            <!-- using SecretManagerSyntaxUtils. Making it optional produces runtime failures -->
-            <!-- of the kind ClassNotFoundError. -->
-            <optional>false</optional>
+            <optional>true</optional>
         </dependency>
 
         <!-- KMS -->

spring-cloud-gcp-autoconfigure/src/main/java/com/google/cloud/spring/autoconfigure/secretmanager/SecretManagerConfigDataLocationResolver.java

@@ -53,14 +53,43 @@ public class SecretManagerConfigDataLocationResolver implements
    */
   private static SecretManagerServiceClient secretManagerServiceClient;
 
+  /**
+   * Checks if the property can be resolved by the Secret Manager resolver.
+   * For the check, we rely on the presence of the SecretManagerSyntaxUtils class, which is an
+   * optional dependency.
+   * Since optional dependencies may not be present at runtime, we explicitly check for its
+   * existence before resolving the property.
+   * If it's not present, it means this config resolver is not meant to be used.
+   *
+   * @return true if it contains the expected `sm@` or `sm://` prefix, false otherwise.
+   */
   @Override
   public boolean isResolvable(ConfigDataLocationResolverContext context,
       ConfigDataLocation location) {
+    boolean secretManagerSyntaxUtilsPresent = isClassPresent("com.google.cloud.spring.secretmanager.SecretManagerSyntaxUtils");
+    if (!secretManagerSyntaxUtilsPresent) {
+      return false;
+    }
     Optional<String> matchedPrefix = getMatchedPrefixes(location::hasPrefix);
     warnIfUsingDeprecatedSyntax(logger, matchedPrefix.orElse(""));
     return matchedPrefix.isPresent();
   }
 
+  /**
+   * Checks if the specified class is present in this runtime.
+   *
+   * @param clazzFullName the full name of the class for the existence check
+   * @return true if present
+   */
+  private boolean isClassPresent(String clazzFullName) {
+    try {
+      Class.forName(clazzFullName);
+      return true;
+    } catch (ClassNotFoundException e) {
+      return false;
+    }
+  }
+
   @Override
   public List<SecretManagerConfigDataResource> resolve(ConfigDataLocationResolverContext context,
       ConfigDataLocation location)