feat: add webhooks and callbacks support

Author: haseeb-mhr

README.md

@@ -50,6 +50,8 @@ Core Interfaces's Maven group ID is `io.apimatic`, and its artifact ID is `core-
 | [`ExceptionCreator`](./src/main/java/io/apimatic/coreinterfaces/type/functional/ExceptionCreator.java)                              | Functional interface to  create the SDK exception                                      |
 | [`Serializer`](./src/main/java/io/apimatic/coreinterfaces/type/functional/Serializer.java)                                          | Functional interface to  apply the serialization function                              |
 | [`ContextInitializer`](./src/main/java/io/apimatic/coreinterfaces/type/functional/ContextInitializer.java)                          | Functional Interface to apply the context initialization function for the response models |
+| [`SignatureVerifier`](./src/main/java/io/apimatic/coreinterfaces/security/SignatureVerifier.java)                                          | Defines a contract for verifying the signature of an HTTP request                              |
+| [`VerificationResult`](./src/main/java/io/apimatic/coreinterfaces/security/VerificationResult.java)                          | Represents the result of an operation that can either succeed or fail with an error message |
 
 ## Enumerations
 

src/main/java/io/apimatic/coreinterfaces/security/SignatureVerifier.java

@@ -0,0 +1,19 @@
+package io.apimatic.coreinterfaces.security;
+
+import io.apimatic.coreinterfaces.http.request.Request;
+
+import java.util.concurrent.CompletableFuture;
+
+/**
+ * Defines a contract for verifying the signature of an HTTP request.
+ */
+public interface SignatureVerifier {
+
+    /**
+     * Verifies the signature of the specified HTTP request.
+     *
+     * @param request The HTTP request data to verify.
+     * @return A {@link CompletableFuture} containing the outcome of the verification process.
+     */
+    CompletableFuture<VerificationResult> verifyAsync(Request request);
+}

src/main/java/io/apimatic/coreinterfaces/security/VerificationResult.java

@@ -0,0 +1,76 @@
+package io.apimatic.coreinterfaces.security;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+/**
+ * Represents the result of an operation that can either succeed
+ * or fail with an error message.
+ */
+public interface VerificationResult {
+
+    /**
+     * Indicates whether the verification succeeded.
+     *
+     * @return true if successful; false otherwise.
+     */
+    boolean isSuccess();
+
+    /**
+     * Gets the collection of error messages, if any.
+     * Always returns a read-only list (never null).
+     *
+     * @return unmodifiable list of errors, empty if success.
+     */
+    List<String> getErrors();
+
+    /**
+     * Creates a successful result.
+     *
+     * @return a success result
+     */
+    static VerificationResult success() {
+        return DefaultImpl.SUCCESS;
+    }
+
+    /**
+     * Creates a failed result with the given error messages.
+     *
+     * @param errors list of error messages
+     * @return a failure result
+     */
+    static VerificationResult failure(String... errors) {
+        return new DefaultImpl(errors);
+    }
+
+    /**
+     * Default hidden implementation inside the interface.
+     */
+    class DefaultImpl implements VerificationResult {
+        private static final VerificationResult SUCCESS = new DefaultImpl();
+
+        private final List<String> errors;
+
+        private DefaultImpl(String... errors) {
+            this.errors = Collections.unmodifiableList(errors != null ?
+                    Arrays.asList(errors) : Collections.emptyList());
+        }
+
+        @Override
+        public boolean isSuccess() {
+            return errors.isEmpty();
+        }
+
+        @Override
+        public List<String> getErrors() {
+            return errors;
+        }
+
+        @Override
+        public String toString() {
+            return isSuccess()
+                    ? "VerificationResult{success}"
+                    : "VerificationResult{errors=" + errors + "}";
+        }
+    }
+}