Add simple redshift_datashare resource

[sworisbreathing]

This is a simplified version of #17. Effectively it's just the auto mode from that PR.

Adds a redshit_datashare resource to manage data sharing between Redshift clusters. This should be defined on the producer cluster.

resource "redshift_datashare" "share" {
  name = "my_datashare"
  owner = "fred" # optional
  publicly_accessible = true # optional. default is false
  schemas = [
    redshift_schema.schema.name,
    redshift_schema.other_schema.name,
  ]
}

For each schema, we ALTER DATASHARE ADD SCHEMA. We also add ALL TABLES IN SCHEMA and ALL FUNCTIONS IN SCHEMA and SET INCLUDENEW=true FOR SCHEMA, so that newly-created tables/functions are automatically exposed to the datashare by the redshift cluster itself, without needing to re-run terraform.

There is test coverage, however because data sharing only works on RA3 clusters, the test case is conditionally enabled on whether or not an environment variable named REDSHIFT_DATASHARE_SUPPORTED is set:

github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:13:      redshiftDatashare                               100.0%        
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:89:      resourceRedshiftDatashareExists                 75.0%
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:105:     resourceRedshiftDatashareCreate                 59.3%
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:153:     addSchemaToDatashare                            75.0%
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:166:     resourceRedshiftDatashareAddSchema              61.5%
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:188:     resourceRedshiftDatashareAddAllFunctions        100.0%        
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:195:     resourceRedshiftDatashareAddAllTables           100.0%        
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:202:     removeSchemaFromDatashare                       75.0%
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:215:     resourceRedshiftDatashareRemoveAllFunctions     100.0%        
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:222:     resourceRedshiftDatashareRemoveAllTables        100.0%        
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:229:     resourceRedshiftDatashareRemoveSchema           50.0%
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:248:     resourceRedshiftDatashareRead                   81.0%
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:293:     readDatashareSchemas                            83.3%
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:319:     resourceRedshiftDatashareUpdate                 61.5%
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:345:     setDatashareOwner                               84.6%
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:366:     setDatasharePubliclyAccessble                   75.0%
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:381:     setDatashareSchemas                             75.0%
github.com/brainly/terraform-provider-redshift/redshift/resource_redshift_datashare.go:411:     resourceRedshiftDatashareDelete                 63.2%

[sworisbreathing]

@winglot if you're happy with this approach it should be pretty quick for me to add a corresponding data source. I'll also need to add a datashare_privilege resource to GRANT USAGE ON DATASHARE to consumer clusters (that's somewhat dependent on #16 since you need to specify the namespace guid for the grant).

[winglot]

Could you move the strings to consts as other resources have?

const (
  datashareNameAttr  = "name"
  datashareOwnerAttr = "owner"
  ...
)

In my opinion, this is a better approach rather than raw strings all around, it eliminates the possibility of a typo and makes it easier to rename an attribute in the future if needed.

[sworisbreathing]

@winglot - I've moved the attribute names to constants and amended the commit.

[sworisbreathing]

Rebased #20 on these changes as well.