Isolate secrets from other extensions

[brichet]

Any extension can requires the SecretsManager or the SecretsConnector and fetch/update the secrets of others extensions namespace.

We should find a way to prevent this.

• the secrets connector should probably be usable only by the manager
• an extension adding a namespace should be the only one able to manage the associated secrets

[brichet]

• the secrets connector should probably be usable only by the manager

The extension providing the connector should register it to the manager, instead of providing a token.
This would avoid exposing it to any third party extension.

const myConnector: JupyterFrontEndPlugin<void> = {
  id: 'my-extension:my-connector',
  description: 'Plugin registering my connector to the secrets manager',
  autoStart: true,
  requires: [ISecretsManager],
  activate: (app: JupyterFrontEnd, manager: ISecretsManager): void => {
    manager.connector = new MyConnector();
  }
};

The manager should allow only one extension to provide a connector, otherwise it could be locked to prevent any at attempt to save passwords from a third party extension.