fix: expose APIFY_USER_IS_PAYING env var to the configuration (#507)

Adds `APIFY_USER_IS_PAYING` environmental variable annotation to the
configuration.

- PR with updated documentation is here:
apify/apify-docs#1706
- PR with changes to the `apify-shared-python` (adding the env variable
to the shared constants):
apify/apify-shared-python#40 - (this is a
prerequisite, and it needs to be merged first, then the package here
needs to be updated as follow-up)

Author: vdusek

src/apify/_configuration.py

@@ -334,6 +334,15 @@ class Configuration(CrawleeConfiguration):
         ),
     ] = None
 
+    user_is_paying: Annotated[
+        bool,
+        Field(
+            alias='apify_user_is_paying',
+            description='True if the user calling the Actor is paying user',
+        ),
+        BeforeValidator(lambda val: False if val == '' else val),
+    ] = False
+
     web_server_port: Annotated[
         int,
         Field(