Add `channel_id` attribute in `slack_notify_service` to avoid Slack rate limiting

[mopp]

Hi 👋
Thanks for the terraform provider for Sentry.

What I've faced

terraform apply have failed due to Slack rate limit error.

Unable to create, got status 400: {"actions":["Slack: Requests to Slack exceeded the rate limit. Please try again later."]}

I can reproduce the error manually on the Sentry web page, also the Sentry official documents shows the way to avoid rate limiting.

If you're attempting to save a Slack alert rule and are receiving the following error: "Requests to slack were rate limited. Please try again later.", you may enter in the channel or user ID in addition to the channel name.

I can create successfully on the web page with Slack channel ID. But terraform apply still causes the same error after few days. It had better to add channel_id attribute in slack_notify_service attribute.

[slobo]

In latest version (0.14.6), when I try to set the channel_id, I get the following error:

│ Cannot set value for this attribute as the provider has marked it as read-only. Remove the configuration line setting the value.

Note, in version 0.11, when actions was still an object, one could easily set the channel_id