(`nullplatform_notification_channel`) feature request: add support for importing resources

[marrws]

Trying importing a notification channel created using your API throws a non support error:

$ terraform import nullplatform_notification_channel.my_resource_name ID123456789
│ Error: resource nullplatform_notification_channel does not support import

Expected behavior

• Provider should allow importing already created notification channels

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Hashicorp docs: https://developer.hashicorp.com/terraform/plugin/sdkv2/resources/import#importer-state-function