Differentiate executor timeouts for builds, sync, and async executions

Adds Executor\Exception\Timeout (with timeoutSeconds) and translates it at
each call site into BUILD_TIMEOUT, FUNCTION_SYNCHRONOUS_TIMEOUT, or
FUNCTION_ASYNCHRONOUS_TIMEOUT instead of always using the misleading sync
function error. Build timeouts now append to streamed buildLogs rather
than replacing them, and the build worker reports its timeout via Span.

src/Executor/Exception.php

@@ -0,0 +1,7 @@
+<?php
+
+namespace Executor;
+
+class Exception extends \Exception
+{
+}