support multiple corpus accounts (#334)

Add support for sharding across multiple storage accounts for blob containers used for corpus management.

Things to note:

1. Additional storage accounts must be in the same resource group, support the "blob" endpoint, and have the tag `storage_type` with the value `corpus`.  A utility is provided (`src/utils/add-corpus-storage-accounts`), which adds storage accounts. 
2. If any secondary storage accounts exist, they are used by default for containers.
3. Storage account names are cached in memory the Azure Function instance forever.   Upon adding new storage accounts, the app needs to be restarted to pick up the new accounts.

src/api-service/__app__/onefuzzlib/notifications/teams.py

@@ -8,6 +8,7 @@ from typing import Any, Dict, List, Optional
 
 import requests
 from onefuzztypes.models import Report, TeamsTemplate
+from onefuzztypes.primitives import Container
 
 from ..azure.containers import auth_download_url
 from ..tasks.config import get_setup_container
@@ -51,7 +52,7 @@ def send_teams_webhook(
 
 
 def notify_teams(
-    config: TeamsTemplate, container: str, filename: str, report: Optional[Report]
+    config: TeamsTemplate, container: Container, filename: str, report: Optional[Report]
 ) -> None:
     text = None
     facts: List[Dict[str, str]] = []