[Identity] Migration guide for JSON-based authentication in azure-common (Azure#16889)

* Basic guidance on migration

* Add section for file-based auth backup

* Fix section naming

* Warn, point to AD docs, and unblock

* Address feedback

* Use KeyVaultManagementClient

* Add base_url and credential_scopes

* Thanks, Laurent!

Author: mccoyp

sdk/identity/azure-identity/migration_guide.md

@@ -0,0 +1,61 @@
+# Guide for migrating to azure-identity from azure-common
+
+## JSON- and file-based authentication
+
+To encourage best security practices, `azure-identity` does not support JSON- and file-based authentication in the same
+way as `azure-common`. `azure-common` provided factory methods like [`get_client_from_json_dict`][client_from_json] and
+[`get_client_from_auth_file`][client_from_auth_file] that are no longer available in `azure-identity`.
+
+In `azure-common` you could provide credentials in a JSON dictionary, or from a JSON file:
+```python
+from azure.common.client_factory import get_client_from_json_dict, get_client_from_auth_file
+from azure.mgmt.keyvault import KeyVaultManagementClient
+# Provide credentials in JSON:
+json_dict = {
+    "clientId": "...",
+    "clientSecret": "...",
+    "subscriptionId": "...",
+    "tenantId": "...",
+    "activeDirectoryEndpointUrl": "https://login.microsoftonline.com",
+    "resourceManagerEndpointUrl": "https://management.azure.com"
+}
+client = get_client_from_json_dict(KeyVaultManagementClient, json_dict)
+# Or, provide credentials from a JSON file:
+client = get_client_from_auth_file(KeyVaultManagementClient, "credentials.json")
+```
+
+If it's not possible to immediately migrate from file-based authentication, you can still use `azure-identity`. With a
+JSON file containing your credentials, you can use [`json.load`][json] to authenticate a service principal with a
+[`ClientSecretCredential`][client_secret_cred]:
+```python
+import json
+from azure.identity import ClientSecretCredential
+from azure.mgmt.keyvault import KeyVaultManagementClient
+
+with open("credentials.json") as json_file:
+    json_dict = json.load(json_file)
+    
+credential = ClientSecretCredential(
+    tenant_id=json_dict["tenantId"],
+    client_id=json_dict["clientId"],
+    client_secret=json_dict["clientSecret"],
+    authority=json_dict["activeDirectoryEndpointUrl"]
+)
+client = KeyVaultManagementClient(
+    credential,
+    json_dict["subscriptionId"],
+    base_url=json_dict["resourceManagerEndpointUrl"],
+    credential_scopes=["{}/.default".format(json_dict["resourceManagerEndpointUrl"])]
+)
+```
+
+If storing credentials in a file, be sure to protect access to this file. Make certain that it's excluded by version
+control -- for example, by adding the credential file name to your project's `.gitignore` file.
+
+The global documentation for authenticating Python apps on Azure is available [here][authenticate_docs].
+
+[authenticate_docs]: https://docs.microsoft.com/azure/developer/python/azure-sdk-authenticate?tabs=cmd
+[client_from_json]: https://docs.microsoft.com/python/api/azure-common/azure.common.client_factory?view=azure-python#get-client-from-json-dict-client-class--config-dict----kwargs-
+[client_from_auth_file]: https://docs.microsoft.com/python/api/azure-common/azure.common.client_factory?view=azure-python#get-client-from-auth-file-client-class--auth-path-none----kwargs-
+[client_secret_cred]: https://docs.microsoft.com/python/api/azure-identity/azure.identity.clientsecretcredential?view=azure-python
+[json]: https://docs.python.org/3/library/json.html#json.load