Force all Optional attributes to Computed

[ewbankkit]

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Closes #191.
Closes #301.
Closes #185.
Closes #220.
Closes #401.
Closes #389.
Closes #362.

This PR addresses the unexpected drift detection reported by Terraform when attributes marked as Optional and with no default value in a resource's CloudFormation schema are not configured by a practitioner but the underlying AWS service applies a default value.
These perpetual diffs are result of a fundamental difference in the way that CloudFormation (and hence the CloudControl API) and Terraform handle unconfigured optional attributes; CloudFormation effectively ignores the attribute but Terraform wants the attribute to have an empty, null value.
To prevent the perpetual diffs we must mark the attributes as Computed when we generate the resource's Terraform schema, indicating that if no value has been configured then Terraform will not perform drift detection on the attribute.

% TF_LOG=ERROR make testacc PKG_NAME=internal/aws/ec2 TESTARGS='-run=TestAccAWSEC2VPC_CidrBlock'
TF_ACC=1 go test ./internal/aws/ec2 -v -count 1 -parallel 20 -run=TestAccAWSEC2VPC_CidrBlock -timeout 180m
=== RUN   TestAccAWSEC2VPC_CidrBlock
=== PAUSE TestAccAWSEC2VPC_CidrBlock
=== CONT  TestAccAWSEC2VPC_CidrBlock
--- PASS: TestAccAWSEC2VPC_CidrBlock (53.13s)
PASS
ok  	github.com/hashicorp/terraform-provider-awscc/internal/aws/ec2	53.994s
% TF_LOG=ERROR make testacc PKG_NAME=internal/aws/iam TESTARGS='-run=TestAccAWSIAMRole_AssumeRolePolicyDocument'
TF_ACC=1 go test ./internal/aws/iam -v -count 1 -parallel 20 -run=TestAccAWSIAMRole_AssumeRolePolicyDocument -timeout 180m
=== RUN   TestAccAWSIAMRole_AssumeRolePolicyDocument
=== PAUSE TestAccAWSIAMRole_AssumeRolePolicyDocument
=== CONT  TestAccAWSIAMRole_AssumeRolePolicyDocument
--- PASS: TestAccAWSIAMRole_AssumeRolePolicyDocument (42.54s)
PASS
ok  	github.com/hashicorp/terraform-provider-awscc/internal/aws/iam	44.481s