management: Add change_auth_backends command.

A utility command to enable or disable certain authentication backends
for a realm from the command line. Can be helpful e.g. if the
administrator accidentally disables some auth methods in the UI leaving
themselves with none remaining that they could actually use to log back
into the organization.

Example usage:
```
(zulip-py3-venv) vagrant@c32c137f59a0:/srv/zulip$ ./manage.py change_auth_backends -r zulip --show
Current authentication backends for the realm:
Enabled backends:
  Dev
  Email
  GitHub
  GitLab
  Google
  Apple
  SAML
  OpenID Connect
(zulip-py3-venv) vagrant@c32c137f59a0:/srv/zulip$ ./manage.py change_auth_backends -r zulip --disable GitHub
Disabling GitHub backend for realm Zulip Dev
Updated authentication backends for the realm:
Enabled backends:
  Dev
  Email
  GitLab
  Google
  Apple
  SAML
  OpenID Connect
Disabled backends:
  GitHub
Done!
(zulip-py3-venv) vagrant@c32c137f59a0:/srv/zulip$ ./manage.py change_auth_backends -r zulip --enable GitHub
Enabling GitHub backend for realm Zulip Dev
Updated authentication backends for the realm:
Enabled backends:
  Dev
  Email
  GitHub
  GitLab
  Google
  Apple
  SAML
  OpenID Connect
Done!
```

zerver/management/commands/change_auth_backends.py

@@ -0,0 +1,85 @@
+from argparse import ArgumentParser
+from typing import Any
+
+from django.core.management.base import CommandError
+from typing_extensions import override
+
+from zerver.actions.realm_settings import do_set_realm_authentication_methods
+from zerver.lib.management import ZulipBaseCommand
+
+
+class Command(ZulipBaseCommand):
+    help = """Enable or disable an authentication backend for a realm"""
+
+    @override
+    def add_arguments(self, parser: ArgumentParser) -> None:
+        self.add_realm_args(parser, required=True)
+        group = parser.add_mutually_exclusive_group(required=True)
+        group.add_argument(
+            "--enable",
+            type=str,
+            help="Name of the authentication backend to enable",
+        )
+        group.add_argument(
+            "--disable",
+            type=str,
+            help="Name of the authentication backend to disable",
+        )
+        group.add_argument(
+            "--show",
+            action="store_true",
+            help="Show current authentication backends for the realm",
+        )
+
+    @override
+    def handle(self, *args: Any, **options: str) -> None:
+        realm = self.get_realm(options)
+        assert realm is not None  # Should be ensured by parser
+
+        auth_methods = realm.authentication_methods_dict()
+
+        if options["show"]:
+            print("Current authentication backends for the realm:")
+            print_auth_methods_dict(auth_methods)
+            return
+
+        if options["enable"]:
+            backend_name = options["enable"]
+            check_backend_name_valid(backend_name, auth_methods)
+
+            auth_methods[backend_name] = True
+            print(f"Enabling {backend_name} backend for realm {realm.name}")
+        elif options["disable"]:
+            backend_name = options["disable"]
+            check_backend_name_valid(backend_name, auth_methods)
+
+            auth_methods[backend_name] = False
+            print(f"Disabling {backend_name} backend for realm {realm.name}")
+
+        do_set_realm_authentication_methods(realm, auth_methods, acting_user=None)
+
+        print("Updated authentication backends for the realm:")
+        print_auth_methods_dict(realm.authentication_methods_dict())
+        print("Done!")
+
+
+def check_backend_name_valid(backend_name: str, auth_methods_dict: dict[str, bool]) -> None:
+    if backend_name not in auth_methods_dict:
+        raise CommandError(
+            f"Backend {backend_name} is not a valid authentication backend. Valid backends: {list(auth_methods_dict.keys())}"
+        )
+
+
+def print_auth_methods_dict(auth_methods: dict[str, bool]) -> None:
+    enabled_backends = [backend for backend, enabled in auth_methods.items() if enabled]
+    disabled_backends = [backend for backend, enabled in auth_methods.items() if not enabled]
+
+    if enabled_backends:
+        print("Enabled backends:")
+        for backend in enabled_backends:
+            print(f"  {backend}")
+
+    if disabled_backends:
+        print("Disabled backends:")
+        for backend in disabled_backends:
+            print(f"  {backend}")