Add import for GRANT permissions

[Imploxion]

Hi.
Actually we are trying to move all the queries that we have in Redshift related to internal configuration of the database (users, groups, schemas and permissions).
Until now we have got no problem with users, groups and schema resources but now as we are trying to move the permissions we tried to use imports as the same way we made with the other resources.

The module id is different as the others ones as it is not using an internal id from redshift but the provider generates it and we tried to generate the id for existing grant permissions as the same way. When we do it we faced an error that the provider does not support import for grant resources.

Error: resource redshift_grant doesn't support import

Could be possible to add the import function for this resources?

[mtesch-um]

This would be a great feature, it'd be extremely helpful for validating terraforming an existing database.

[franknazario]

+1