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/tasks/config.py

@@ -10,15 +10,12 @@ from uuid import UUID
 
 from onefuzztypes.enums import Compare, ContainerPermission, ContainerType, TaskFeature
 from onefuzztypes.models import TaskConfig, TaskDefinition, TaskUnitConfig
+from onefuzztypes.primitives import Container
 
-from ..azure.containers import (
-    StorageType,
-    blob_exists,
-    container_exists,
-    get_container_sas_url,
-)
+from ..azure.containers import blob_exists, container_exists, get_container_sas_url
 from ..azure.creds import get_instance_id, get_instance_url
 from ..azure.queue import get_queue_sas
+from ..azure.storage import StorageType
 from .defs import TASK_DEFINITIONS
 
 LOGGER = logging.getLogger("onefuzz")
@@ -334,7 +331,7 @@ def build_task_config(
     return config
 
 
-def get_setup_container(config: TaskConfig) -> str:
+def get_setup_container(config: TaskConfig) -> Container:
     for container in config.containers:
         if container.type == ContainerType.setup:
             return container.name

src/api-service/__app__/onefuzzlib/tasks/main.py

@@ -18,9 +18,9 @@ from onefuzztypes.webhooks import (
     WebhookEventTaskStopped,
 )
 
-from ..azure.containers import StorageType
 from ..azure.image import get_os
 from ..azure.queue import create_queue, delete_queue
+from ..azure.storage import StorageType
 from ..orm import MappingIntStrAny, ORMMixin, QueryFilter
 from ..pools import Node, Pool, Scaleset
 from ..proxy_forward import ProxyForward

src/api-service/__app__/onefuzzlib/tasks/scheduler.py

@@ -11,7 +11,8 @@ from onefuzztypes.enums import OS, PoolState, TaskState
 from onefuzztypes.models import WorkSet, WorkUnit
 from pydantic import BaseModel
 
-from ..azure.containers import StorageType, blob_exists, get_container_sas_url
+from ..azure.containers import blob_exists, get_container_sas_url
+from ..azure.storage import StorageType
 from ..pools import Pool
 from .config import build_task_config, get_setup_container
 from .main import Task