BotService slack channel field registerBeforeOAuthFlow incorrectly marked read-only, which breaks saving data

When creating or updating bot service slack channels, the `registerBeforeOAuthFlow` field must be set to true in order for the client id and secrets data to be saved. This field is incorrectly marked read-only in the spec, so generated clients don't include it when creating/updating slack channels, causing the clientId, clientSecret, signingSecret and validationToken fields to be ignored on the server side. This breaks all clients trying to automate creating slack channels, and also breaks the `az bot slack` cli.

https://github.com/Azure/azure-rest-api-specs/blob/bcec16b1ef77dc266ada0b3eb6d86eae0cdf3b16/specification/botservice/resource-manager/Microsoft.BotService/stable/2021-03-01/botservice.json#L2105-L2109

Open specification/botservice/resource-manager/Microsoft.BotService/stable/2021-03-01/botservice.json at lines 2105-2109 and inspect the read-only setting for registerBeforeOAuthFlow. Done means generated create/update Slack channel clients include this field, allowing clientId, clientSecret, signingSecret, and validationToken to be saved through the API and az bot slack.

Written by the indexing model from the issue text.