Store and pull tenant creds from Key Vault.

The tenant ID should be hashed and used as the key for the JSON blob of
relevant creds for any given tenant. Azure CSP interface methods that
need to source creds should call the internal `_source_creds` method,
either with a `tenant_id` or no parameters. That method will source the
creds. If a tenant ID is provided, it will source them from the Key
Vault. If not provided, it will return the default creds for the app
registration in the home tenant.

atst/utils/__init__.py

@@ -1,3 +1,4 @@
+import hashlib
 import re
 
 from sqlalchemy.exc import IntegrityError
@@ -41,3 +42,8 @@ def commit_or_raise_already_exists_error(message):
     except IntegrityError:
         db.session.rollback()
         raise AlreadyExistsError(message)
+
+
+def sha256_hex(string):
+    hsh = hashlib.sha256(string.encode())
+    return hsh.digest().hex()