resource/aws_vpc_ipv4_cidr_block_association: Support resource import

[bflad]

Changes proposed in this pull request:

• Use DescribeVpcs with cidr-block-association.association-id filter in read function
• Add Importer to resource schema

Output from acceptance testing:

$ make testacc TEST=./aws TESTARGS='-run=TestAccAwsVpcIpv4CidrBlockAssociation_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAwsVpcIpv4CidrBlockAssociation_basic -timeout 120m
=== RUN   TestAccAwsVpcIpv4CidrBlockAssociation_basic
--- PASS: TestAccAwsVpcIpv4CidrBlockAssociation_basic (49.45s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	49.493s

[bflad]

@ewbankkit if you have a second to verify this as well, we can get this in 👍

[tombuildsstuff]

LGTM 👍

[ewbankkit]

@bflad I’m hands off keyboard for rest of this week so can’t run the acceptance tests but the code changes LGTM.

[bflad]

This has been released in version 1.27.0 of the AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

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