provider/aws: Add support for CloudTrail

[radeksimko]

Test plan

$ make testacc TEST=./builtin/providers/aws TESTARGS='-run=CloudTrail' 2>/dev/null

go generate ./...
TF_ACC=1 go test ./builtin/providers/aws -v -run=CloudTrail -timeout 90m
=== RUN TestAccAWSCloudTrail_basic
--- PASS: TestAccAWSCloudTrail_basic (16.34s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    16.353s

I wanted to add some more complicated examples into acc. tests, e.g. with SNS, but I struggled to get the policy right and we don't have support for CloudWatch Log Groups yet either.

[catsby]

This too looks great, thanks @radeksimko ! Merge away

[ghost]

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