feat: add flag to disable password auth (#5991)

Adds a flag --disable-password-auth that prevents the password login
endpoint from working unless the user has the "owner" (aka. site admin)
role.

Adds a subcommand `coder server create-admin-user` which creates a user
directly in the database with the "owner" role, the "admin" role in
every organization, and password auth. This is to avoid lock-out
situations where all accounts have the login type set to an identity
provider and nobody can login.

docs/api/general.md

@@ -289,6 +289,17 @@ curl -X GET http://coder-server:8080/api/v2/config/deployment \
       }
     }
   },
+  "disable_password_auth": {
+    "default": true,
+    "enterprise": true,
+    "flag": "string",
+    "hidden": true,
+    "name": "string",
+    "secret": true,
+    "shorthand": "string",
+    "usage": "string",
+    "value": true
+  },
   "disable_path_apps": {
     "default": true,
     "enterprise": true,