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

README.md

@@ -51,6 +51,26 @@ C:\php-sdk\php-<version>\vc15\x64\php-src
 $ nmake test-awscrt
 ```
 
+## Debugging
+Using [PHPBrew](https://github.com/phpbrew/phpbrew) to build/manage multiple versions of PHP is helpful.
+
+Note: You must use a debug build of PHP to debug native extensions. 
+See the [PHP Internals Book](https://www.phpinternalsbook.com/php7/build_system/building_php.html) for more info
+
+```shell
+# PHP 8 example
+$ phpbrew install --stdout -j 8 8.0 +default -- CFLAGS=-Wno-error --disable-cgi --enable-debug
+# PHP 5.5 example
+$ phpbrew install --stdout -j 8 5.5 +default -openssl -mbstring -- CFLAGS="-w -Wno-error" --enable-debug --with-zlib=/usr/local/opt/zlib
+$ phpbrew switch php-8.0.6 # or whatever version is current, it'll be at the end of the build output
+$ phpize
+$ ./configure
+$ make CMAKE_BUILD_TYPE=Debug
+```
+
+Ensure that the php you launch from your debugger is the result of `which php`, not just
+the system default php.
+
 ## Security
 
 See [CONTRIBUTING](CONTRIBUTING.md#security-issue-notifications) for more information.