[management] Add option to disable default all-to-all policy (#3970)

This PR introduces a new configuration option `DisableDefaultPolicy` that prevents the creation of the default all-to-all policy when new accounts are created. This is useful for automation scenarios where explicit policies are preferred.
### Key Changes:
- Added DisableDefaultPolicy flag to the management server config
- Modified account creation logic to respect this flag
- Updated all test cases to explicitly pass the flag (defaulting to false to maintain backward compatibility)
- Propagated the flag through the account manager initialization chain

### Testing:

- Verified default behavior remains unchanged when flag is false
- Confirmed no default policy is created when flag is true
- All existing tests pass with the new parameter

infrastructure_files/management.json.tmpl

@@ -38,6 +38,7 @@
              "0.0.0.0/0"
         ]
     },
+    "DisableDefaultPolicy": $NETBIRD_MGMT_DISABLE_DEFAULT_POLICY,
     "Datadir": "",
     "DataStoreEncryptionKey": "$NETBIRD_DATASTORE_ENC_KEY",
     "StoreConfig": {