resource/aws_cloudfront_distribution: Validate *_cache_behavior forwarded_values cookies configuration block forward argument

[bflad]

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Closes #8493

Release note for CHANGELOG:

resource/aws_cloudfront_distribution: Validate `*_cache_behavior` `forwarded_values` `cookies` configuration block `forward` argument

This also prevents panics with an additional nil check, which bubbles up the API error should the attribute validation be removed for some reason. Verified the panic by changing an existing test configuration to forward = "" without code updates:

=== CONT  TestAccAWSCloudFrontDistribution_S3Origin
panic: interface conversion: interface {} is nil, not map[string]interface {}

goroutine 668 [running]:
github.com/terraform-providers/terraform-provider-aws/aws.expandForwardedValues(0xc000cd72f0, 0xc000cd7170)
  /Users/bflad/src/github.com/terraform-providers/terraform-provider-aws/aws/cloudfront_distribution_configuration_structure.go:418 +0x515
github.com/terraform-providers/terraform-provider-aws/aws.expandCloudFrontDefaultCacheBehavior(0xc000cd7170, 0x5e9a943)
  /Users/bflad/src/github.com/terraform-providers/terraform-provider-aws/aws/cloudfront_distribution_configuration_structure.go:196 +0x21b
github.com/terraform-providers/terraform-provider-aws/aws.expandDistributionConfig(0xc00039fe30, 0xc000a89f20)
  /Users/bflad/src/github.com/terraform-providers/terraform-provider-aws/aws/cloudfront_distribution_configuration_structure.go:39 +0x23a
github.com/terraform-providers/terraform-provider-aws/aws.resourceAwsCloudFrontDistributionCreate(0xc00039fe30, 0x4fd92c0, 0xc0004b7000, 0x2, 0xa49a740)
  /Users/bflad/src/github.com/terraform-providers/terraform-provider-aws/aws/resource_aws_cloudfront_distribution.go:740 +0x6c

With the nil check in place, the API error is returned:

--- FAIL: TestAccAWSCloudFrontDistribution_S3Origin (18.14s)
    testing.go:568: Step 0 error: errors during apply:

        Error: error creating CloudFront Distribution: InvalidParameter: 1 validation error(s) found.
        - missing required field, CreateDistributionWithTagsInput.DistributionConfigWithTags.DistributionConfig.DefaultCacheBehavior.ForwardedValues.Cookies.

By adding ValidateFunc we can prevent this issue during planning:

--- FAIL: TestAccAWSCloudFrontDistribution_S3Origin (1.73s)
    testing.go:568: Step 0 error: config is invalid: expected default_cache_behavior.0.forwarded_values.0.cookies.0.forward to be one of [all none whitelist], got

Output from acceptance testing:

--- PASS: TestAccAWSCloudFrontDistribution_Origin_EmptyOriginID (1.90s)
--- PASS: TestAccAWSCloudFrontDistribution_Origin_EmptyDomainName (2.03s)
--- PASS: TestAccAWSCloudFrontDistribution_DefaultCacheBehavior_ForwardedValues_Headers (604.75s)
--- PASS: TestAccAWSCloudFrontDistribution_OrderedCacheBehavior_ForwardedValues_Headers (605.93s)
--- PASS: TestAccAWSCloudFrontDistribution_OrderedCacheBehavior_ForwardedValues_Cookies_WhitelistedNames (604.23s)
--- PASS: TestAccAWSCloudFrontDistribution_DefaultCacheBehavior_ForwardedValues_Cookies_WhitelistedNames (606.72s)
--- PASS: TestAccAWSCloudFrontDistribution_disappears (697.81s)
--- PASS: TestAccAWSCloudFrontDistribution_IsIPV6EnabledConfig (2383.17s)
--- PASS: TestAccAWSCloudFrontDistribution_noCustomErrorResponseConfig (2384.35s)
--- PASS: TestAccAWSCloudFrontDistribution_RetainOnDelete (2384.95s)
--- PASS: TestAccAWSCloudFrontDistribution_OriginGroups (2385.23s)
--- PASS: TestAccAWSCloudFrontDistribution_orderedCacheBehavior (2386.96s)
--- PASS: TestAccAWSCloudFrontDistribution_multiOrigin (2387.28s)
--- PASS: TestAccAWSCloudFrontDistribution_WaitForDeployment (2387.45s)
--- PASS: TestAccAWSCloudFrontDistribution_S3Origin (2387.61s)
--- PASS: TestAccAWSCloudFrontDistribution_HTTP11Config (2390.63s)
--- PASS: TestAccAWSCloudFrontDistribution_customOrigin (2392.71s)
--- PASS: TestAccAWSCloudFrontDistribution_ViewerCertificate_AcmCertificateArn_ConflictsWithCloudFrontDefaultCertificate (2391.49s)
--- PASS: TestAccAWSCloudFrontDistribution_noOptionalItemsConfig (2393.55s)
--- PASS: TestAccAWSCloudFrontDistribution_ViewerCertificate_AcmCertificateArn (2397.30s)
--- PASS: TestAccAWSCloudFrontDistribution_S3OriginWithTags (2956.22s)
--- PASS: TestAccAWSCloudFrontDistribution_Enabled (2959.58s)

[bflad]

This has been released in version 2.10.0 of the Terraform AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

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