Cache JobDefinition.get_job_index (#23722)

Summary:
This can take seconds on large jobs, and we call it at twice when
loading a new code location when using defer_snapshots=True - once when
generating the IDs for the JobRefs,and then again when loading the
actual jobs.

Test Plan: BK, verify when running an agent locally that this is called
only once per job when loading a new code locatoin

## Summary & Motivation

## How I Tested These Changes

python_modules/dagster/dagster/_core/definitions/job_definition.py

@@ -56,6 +56,7 @@
 from dagster._core.types.dagster_type import DagsterType
 from dagster._core.utils import str_format_set
 from dagster._utils import IHasInternalInit
+from dagster._utils.cached_method import cached_method
 from dagster._utils.merger import merge_dicts
 
 from .asset_layer import AssetLayer
@@ -979,6 +980,7 @@ def get_config_schema_snapshot(self) -> "ConfigSchemaSnapshot":
     def get_job_snapshot(self) -> "JobSnapshot":
         return self.get_job_index().job_snapshot
 
+    @cached_method
     def get_job_index(self) -> "JobIndex":
         from dagster._core.remote_representation import JobIndex
         from dagster._core.snap import JobSnapshot