Introduces Bytes class (#1019)

This introduces a new `Bytes` standard library class, for working with
binary data.

* Add Bytes class to the standard library
* Change CLI to eval `output.bytes`
* Change code generators to map Bytes to respective underlying type
* Add subscript and concat operator support
* Add binary encoding for Bytes
* Add PCF and Plist rendering for Bytes

Co-authored-by: Kushal Pisavadia <kushi.p@gmail.com>

pkl-gradle/src/main/java/org/pkl/gradle/task/EvalTask.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -51,6 +51,7 @@ public abstract class EvalTask extends ModulesTask {
   public abstract Property<String> getModuleOutputSeparator();
 
   @Input
+  @Optional
   public abstract Property<String> getExpression();
 
   private final Provider<CliEvaluator> cliEvaluator =
@@ -65,7 +66,7 @@ public abstract class EvalTask extends ModulesTask {
                           getModuleOutputSeparator().get(),
                           mapAndGetOrNull(
                               getMultipleFileOutputDir(), it -> it.getAsFile().getAbsolutePath()),
-                          getExpression().get())));
+                          getExpression().getOrNull())));
 
   @SuppressWarnings("unused")
   @OutputFiles