fix: add API key header support in Subscan requests

Author: galvanizze

blockapi/v2/api/subscan.py

@@ -127,6 +127,9 @@ def _yield_staking_rewards(self, address: str) -> Iterable[Decimal]:
 
     def _post(self, request_method: str, body):
         headers = {'Content-Type': 'application/json'}
+        if self.api_key:
+            headers['X-API-Key'] = self.api_key
+
         return self.post(request_method, body=body, headers=headers)
 
     def _opt_raise_on_other_error(self, response: Response) -> None: