Added support for multi tenant authentication (#746)

## Summary of the Pull Request

_What is this about?_

## PR Checklist
* [x] Applies to work item: #562 
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/onefuzz) and sign the CLI.
* [x] Tests added/passed
* [ ] Requires documentation to be updated
* [x] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

## Info on Pull Request

The end-to-end changes needed to have onefuzz deployed with multi-tenant authentication.

## Validation Steps Performed

_How does someone test & validate?_

src/api-service/__app__/onefuzzlib/extension.py

@@ -113,6 +113,10 @@ def build_pool_config(pool: Pool) -> str:
         instance_id=get_instance_id(),
     )
 
+    multi_tenant_domain = os.environ.get("MULTI_TENANT_DOMAIN")
+    if multi_tenant_domain:
+        config.multi_tenant_domain = multi_tenant_domain
+
     filename = f"{pool.name}/config.json"
 
     save_blob(