Bootstrap AST validators in ValidatorRegistry.get_instance()

The query-language REGISTRY snapshots ValidatorRegistry.get_instance()
at import time. This happens during engine bootstrap before
bootstrap_ast_validators() populates the singleton, resulting in an
empty REGISTRY and KeyError in DruidQueryTransformer on UDF queries.

Call bootstrap_ast_validators() in get_instance() when creating the
singleton, matching the pattern used by smite's ValidatorRegistry.
The import is deferred to avoid circular imports with plugin_manager.

Author: cmttt

osprey_worker/src/osprey/engine/ast_validator/validator_registry.py

@@ -16,6 +16,9 @@ def get_instance(cls) -> 'ValidatorRegistry':
         tests, you can create your own validator registry, to only run a subset of validations."""
         if cls._instance is None:
             cls._instance = ValidatorRegistry()
+            from osprey.worker.adaptor.plugin_manager import bootstrap_ast_validators
+
+            bootstrap_ast_validators()
 
         return cls._instance