Add 'arn' attribute to 'aws_redshift_service_account' data source

[ewbankkit]

Similar to hashicorp/terraform#8700 for the aws_elb_service_account data source, add the arn attribute to the aws_redshift_service_account data source.
Acceptance tests:

make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSRedshiftServiceAccount_'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -run=TestAccAWSRedshiftServiceAccount_ -timeout 120m
=== RUN   TestAccAWSRedshiftServiceAccount_basic
--- PASS: TestAccAWSRedshiftServiceAccount_basic (31.19s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	32.215s

[ewbankkit]

arn already included for aws_cloudtrail_service_account data source in #1774.

[stack72]

I don't think we should use fmt.Sprintf here - I think we should use the ARN package as part of AWS

[ewbankkit]

Will do

[ewbankkit]

Added arn.go and arn_test.go:

go test ./aws/ -run=TestArn_
ok  	github.com/terraform-providers/terraform-provider-aws/aws	0.934s

make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSRedshiftServiceAccount_'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -run=TestAccAWSRedshiftServiceAccount_ -timeout 120m
=== RUN   TestAccAWSRedshiftServiceAccount_basic
--- PASS: TestAccAWSRedshiftServiceAccount_basic (44.71s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	45.793s

[stack72]

@ewbankkit this looks much nicer without the fmt.Sprintf :)

[ewbankkit]

@stack72 Anything more you need me to do on this one?

[radeksimko]

LGTM, thanks @ewbankkit

[ghost]

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