Add Discord webhook notifications for plugin downloads (#157)

[Zoriot]

Implement basic Discord webhook support to send notifications when plugins are successfully downloaded. Uses jdwebhooks 2.0.0 library with Discord's Components V2 API.

Changes:

• Add jdwebhooks 2.0.0 dependency to common/impl
• Create DiscordWebhookNotifier class for sending webhook notifications
• Add discord-webhook configuration section to config.yml (disabled by default)
• Update ConfigManager to load and reload webhook settings
• Integrate webhook notifications in UpdateHandler on successful downloads
• Update UpdaterImpl to initialize and manage webhook notifier lifecycle

Webhook notifications include:

• Plugin name
• Version change (old → new)
• Update type (MAJOR, MINOR, PATCH, etc.)
• Changelog link (if available)

Infos

Users can enable by setting discord-webhook.enabled to true and providing a webhook-url in config.yml.

Closes #157

• Config currently does not update automatically
• This implementation set both Avatar + Users - we could also not set that and use it from the Webhook.
• Currently it was only implemented for Updates, Update available might be a feature i contribute

AI Disclosure

GitHub Copilot was used for most of the implementation, all code was manually verified & tested on paper. Additional Changes was made to ensure it works correctly.