Allow AWS endpoint URL to be customized

If EXODUS_AWS_ENDPOINT_URL env var is set, we'll use that as the AWS
endpoint URL when constructing boto clients.

This will never be used in production. The motivation is only to enable
testing against an instance of localstack.

exodus_lambda/functions/origin_request.py

@@ -12,6 +12,12 @@
 
 CONF_FILE = os.environ.get("EXODUS_LAMBDA_CONF_FILE") or "lambda_config.json"
 
+# Endpoint for AWS services.
+# Normally, should be None.
+# You might want to try e.g. "https://localhost:3377" if you want to test
+# this code against localstack.
+ENDPOINT_URL = os.environ.get("EXODUS_AWS_ENDPOINT_URL") or None
+
 
 class OriginRequest(LambdaBase):
     def __init__(self, conf_file=CONF_FILE):
@@ -29,15 +35,21 @@ def __init__(self, conf_file=CONF_FILE):
     @property
     def db_client(self):
         if not self._db_client:
-            self._db_client = boto3.client("dynamodb", region_name=self.region)
+            self._db_client = boto3.client(
+                "dynamodb",
+                region_name=self.region,
+                endpoint_url=ENDPOINT_URL,
+            )
 
         return self._db_client
 
     @property
     def sm_client(self):
         if not self._sm_client:
             self._sm_client = boto3.client(
-                "secretsmanager", region_name=self.region
+                "secretsmanager",
+                region_name=self.region,
+                endpoint_url=ENDPOINT_URL,
             )
 
         return self._sm_client