Implemented SigV4A (#40)

* Implemented should_sign_header callback

* Added SigV4A to SigningAlgorithm

* Added SigV4A test

* Added debugging info to README

* Fixed memory leak from argument vectors

* Fixed memory leak of stream read buffers

* Added support for callables as option values

* Added should_sign_header support and test

* Fixed ELG type bug exposed by aws-crt-ffi change

* Updated to aws-crt-ffi v0.2.0

ext/php_util.c

@@ -0,0 +1,33 @@
+
+/**
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0.
+ */
+
+#include "php_aws_crt.h"
+
+zval *aws_php_zval_new(void) {
+    return emalloc(sizeof(zval));
+}
+
+void aws_php_zval_dtor(void *zval_ptr) {
+    zval *z = zval_ptr;
+    zval_dtor(z);
+    efree(z);
+}
+
+bool aws_php_zval_as_bool(zval *z) {
+#if AWS_PHP_AT_LEAST_7
+    return (Z_TYPE_P(z) == IS_TRUE);
+#else
+    return (Z_TYPE_P(z) == IS_BOOL && Z_LVAL_P(z) != 0);
+#endif
+}
+
+void aws_php_zval_copy(zval *dest, zval *src) {
+#if AWS_PHP_AT_LEAST_7
+    ZVAL_COPY(dest, src);
+#else
+    ZVAL_COPY_VALUE(dest, src);
+#endif
+}