Add ARN to cognito_identity_pool

[bespokebob]

Cognito Identity Pools, like most AWS resources, have an ARN in addition to their ID that is used by other AWS services as a reference.

Fixes #4703

Changes proposed in this pull request:

• Add arn attribute to cognito_identity_pool

Output from acceptance testing:

$ make testacc TEST=./aws TESTARGS='-run=TestAccAWSCognitoIdentityPool_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAWSCognitoIdentityPool_basic -timeout 120m
=== RUN   TestAccAWSCognitoIdentityPool_basic
--- PASS: TestAccAWSCognitoIdentityPool_basic (23.18s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	33.956s

[bflad]

Looks great, thanks @youngbob! 🚀

6 tests passed (all tests)
=== RUN   TestAccAWSCognitoIdentityPool_importBasic
--- PASS: TestAccAWSCognitoIdentityPool_importBasic (7.18s)
=== RUN   TestAccAWSCognitoIdentityPool_basic
--- PASS: TestAccAWSCognitoIdentityPool_basic (10.94s)
=== RUN   TestAccAWSCognitoIdentityPool_openidConnectProviderArns
--- PASS: TestAccAWSCognitoIdentityPool_openidConnectProviderArns (15.33s)
=== RUN   TestAccAWSCognitoIdentityPool_cognitoIdentityProviders
--- PASS: TestAccAWSCognitoIdentityPool_cognitoIdentityProviders (15.44s)
=== RUN   TestAccAWSCognitoIdentityPool_supportedLoginProviders
--- PASS: TestAccAWSCognitoIdentityPool_supportedLoginProviders (15.63s)
=== RUN   TestAccAWSCognitoIdentityPool_samlProviderArns
--- PASS: TestAccAWSCognitoIdentityPool_samlProviderArns (19.61s)

[bflad]

This has been released in version 1.22.0 of the AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!