Add reference implementation for async attribute resolution.

Wraps the basic evaluator to coordinate resolving relevant unknowns as they are identified then advancing evaluation.

PiperOrigin-RevId: 510464285

Author: l46kok

runtime/src/main/java/dev/cel/runtime/CelVariableResolver.java

@@ -14,11 +14,9 @@
 
 package dev.cel.runtime;
 
-import javax.annotation.concurrent.ThreadSafe;
 import java.util.Optional;
 
 /** Functional interface that exposes a method to find a CEL variable value by name. */
-@ThreadSafe
 @FunctionalInterface
 public interface CelVariableResolver {