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/proxy.py

@@ -16,14 +16,15 @@ from onefuzztypes.models import (
     ProxyConfig,
     ProxyHeartbeat,
 )
-from onefuzztypes.primitives import Region
+from onefuzztypes.primitives import Container, Region
 from pydantic import Field
 
 from .__version__ import __version__
 from .azure.auth import build_auth
-from .azure.containers import StorageType, get_file_sas_url, save_blob
+from .azure.containers import get_file_sas_url, save_blob
 from .azure.ip import get_public_ip
 from .azure.queue import get_queue_sas
+from .azure.storage import StorageType
 from .azure.vm import VM
 from .extension import proxy_manager_extensions
 from .orm import MappingIntStrAny, ORMMixin, QueryFilter
@@ -188,7 +189,7 @@ class Proxy(ORMMixin):
         forwards = self.get_forwards()
         proxy_config = ProxyConfig(
             url=get_file_sas_url(
-                "proxy-configs",
+                Container("proxy-configs"),
                 "%s/config.json" % self.region,
                 StorageType.config,
                 read=True,
@@ -203,7 +204,7 @@ class Proxy(ORMMixin):
         )
 
         save_blob(
-            "proxy-configs",
+            Container("proxy-configs"),
             "%s/config.json" % self.region,
             proxy_config.json(),
             StorageType.config,