fix: include jumpstart/region_config.json in MANIFEST.in (#5605)

The file src/sagemaker/jumpstart/region_config.json is missing from the
source distribution. The jumpstart constants module loads this file at
import time and logs an ERROR when it is absent, which triggers
monitoring alerts (e.g. Datadog) in production environments.

Fixes #5550

Author: jam-jee

MANIFEST.in

@@ -1,6 +1,7 @@
 recursive-include src/sagemaker *.py
 
 include src/sagemaker/image_uri_config/*.json
+include src/sagemaker/jumpstart/region_config.json
 include src/sagemaker/pytorch/training_recipes.json
 include src/sagemaker/serve/schema/*.json
 include src/sagemaker/serve/requirements.txt