Support is_read_only in slack.Conversation

[GGonryun]

Description

Slack allows custom channel permissions which may prevent certain users or bots from posting to that channel. An existing property is_read_only from the API is currently missing from slack.Converstations.

(Optional) Slack's documentation

https://api.slack.com/methods/conversations.info#examples
https://api.slack.com/types/conversation

{
    "ok": true,
    "channel": {
        "id": "C012AB3CD",
        "name": "general",
        "is_channel": true,
        // ... other properties
        "is_read_only": false, 
        // ... other properties
    }
}