Implemented async callbacks to PHP regardless of thread model, Sigv4 signing (#24)

* Updated to get latest aws-crt-ffi

* Added implementation for signing_result

* Added main thread queue and callback queueing to signing process

* Updated to get support for date on signing_config_aws

* added date support to SigningConfigAWS, fixed up reference issues on http message signing

* Broke C code into separate files, moved common stuff to php_aws_crt.h

* Use a unity build to make PHP do the right thing with our lib

* Updated aws-crt-ffi to v0.1.3 to get sigv4 changes

* Added CRT::isAvailable for safe runtime detection of CRT

* Fixed up test resource safety, added tests to prove it

* Added CI jobs for every version of PHP on linux (#26)

ext/awscrt.stub.php

@@ -103,6 +103,8 @@ function aws_crt_signing_config_aws_set_signed_body_header_type(int $config, int
 
 function aws_crt_signing_config_aws_set_expiration_in_seconds(int $config, int $expiration_in_seconds): void {}
 
+function aws_crt_signing_config_aws_set_date(int $config, int $timestamp): void {}
+
 function aws_crt_signable_new_from_http_request(int $http_message): int {}
 
 function aws_crt_signable_new_from_chunk(int $input_stream, string $previous_signature): int {}
@@ -111,4 +113,8 @@ function aws_crt_signable_new_from_canonical_request(string $request): int {}
 
 function aws_crt_signable_release(int $signable): void {}
 
+function aws_crt_signing_result_release(int $signing_result): void {}
+
+function aws_crt_signing_result_apply_to_http_request(object $signing_result, object $http_request): void {}
+
 function aws_crt_sign_request_aws(int $signable, int $signing_config, object $on_complete, object $user_data): int {}