fix(notification-channels): add support to Slack private channels

[akahenry]

This PR is an attempt to fix the error described on this issue.

Considering that the isPrivateChannel (together with the privateChannelUrl) field from the Sysdig Create Notification Channel API isn't supported by this Terraform provider, I'm adding it with this PR hoping that this will be sufficient to fix the error.

I didn't test myself the changes because I don't really have an environment where I could do such a thing. However, if you have any suggestions on how I could achieve/build one for that I would appreciate it.

[dbonf]

thanks for this contribution, originally we were on the fence on supporting private slack channels in terraform (see some work on the old branch https://github.com/sysdiglabs/terraform-provider-sysdig/tree/slack-support-private-channel) as this is a peculiar feature, but i can see the value of having it when importing existing notification channels. I took the liberty to add to your work and add some refinements.

[akahenry]

thanks for this contribution, originally we were on the fence on supporting private slack channels in terraform (see some work on the old branch https://github.com/sysdiglabs/terraform-provider-sysdig/tree/slack-support-private-channel) as this is a peculiar feature, but i can see the value of having it when importing existing notification channels. I took the liberty to add to your work and add some refinements.

That's awesome! Thank you all for the quick reply and solution.