Add nv-websocket-client backend SocketModeClient

[seratch]

https://github.com/TakahikoKawasaki/nv-websocket-client looks well-maintained and there are at least 450+ public projects using it as of today. We can consider adding this library support as the 3rd backend for SocketModeClient.

The main reason why I came to think of this addition is to have more proxy supported options for enterprise customers. Unfortunately, Java-WebSocket library does not offer complete proxy support. Refer to the following links for more details:

• SocketModeApp (JavaWebSocket) does not support proxy with username:password #792 (comment)
• https://stackoverflow.com/questions/29430503/java-websocket-with-proxy

Suggesting Tyrus as the only option for enterprise customers with proxy is okay but we want to provide alternatives as much as possible (that's why we added Java-WebSocket library support).

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-socket-mode (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

Please make sure if this topic is specific to this SDK. For general questions/issues about Slack API platform or its server-side, could you submit questions at https://my.slack.com/help/requests/new instead. 🙇

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[seratch]

It seems that there have been no updates to the library in the past three years. I don't believe the project is likely to be terminated, but it would make sense to deprioritize this task for now. I am going to close this issue, but should the project be revitalized in the future, we may revisit this task.