Improve third‑party plugin path handling & plugin management UX

[ThomasMalletCodra]

Is your feature request related to a problem? Please describe.

DataLab’s current plugin workflow could be improved to make third‑party plugin discovery/configuration easier and faster to troubleshoot:

• It’s not clear/easy to point DataLab to multiple external plugin locations.
• From the Plugins configuration dialog, it’s not straightforward to open a plugin’s folder/script for inspection or editing.
• There is no session-based indicator of when plugins were last (re)loaded, which makes it harder to confirm that changes have taken effect.
• Accessing the user configuration directory requires manual navigation, which is inconvenient for support/debug scenarios.
• Allow plugin-specific configuration options

Describe the solution you'd like

1. Allow multiple directories in the environment variable for third‑party plugin paths
   Enable defining one or more folders in the environment variable used as the search path for external plugins (i.e., a “plugin path” env var that supports a list of directories).

2. Add quick-access buttons per plugin in the Plugins configuration dialog
   In the plugin configuration dialog, add buttons/controls to:

   • Open the plugin directory in the file explorer
   • Open the plugin script/file (or reveal it in explorer, depending on platform conventions)

3. Display “last load/reload” timestamp in the Plugins configuration dialog (session-based)
   Add a field showing the date and time of the last plugin load or reload, reset at each application session (i.e., not persisted across restarts).

4. Add a menu action to open the DataLab user configuration directory
   Add a menu item (for example under File) that opens the DataLab user config folder in the system file explorer.

5. Allow plugin-specific configuration options
   Add support for plugin-specific settings that are managed independently from DataLab’s global configuration and are not stored in DataLab’s main configuration system. This would also fit the planned evolution of the configuration backend, which is expected to rely on the SigimaX configuration framework.

Additional context

• These changes aim to improve usability for plugin developers/users and simplify support/debugging.
• Expected behavior should be cross-platform (Windows/macOS/Linux) for path lists and “open folder” actions.
• This feature is also aligned with the planned evolution of DataLab’s configuration system toward the SigimaX configuration framework, which should make it easier to support plugin-specific settings outside DataLab’s global configuration.