adds 'purpose' to kms_crypto_key

[gracenoah]

Fixes hashicorp/terraform-provider-google#3039

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[gracenoah]

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[danawillow]

/cc @drebes

FYI this is already being taken care of in this other open PR: #1856

If you'd like to do the work in this one as well (since I don't know how long it'll take for the other to be finished), I'd ask to please:

• update the read function so the field is set in state on read
• add the field to a test in https://github.com/GoogleCloudPlatform/magic-modules/blob/master/third_party/terraform/tests/resource_kms_crypto_key_test.go (feel free to use the _basic test or create a new one)
• update the docs for the resource (https://github.com/GoogleCloudPlatform/magic-modules/blob/master/third_party/terraform/website/docs/r/google_kms_crypto_key.html.markdown) and datasource (https://github.com/GoogleCloudPlatform/magic-modules/blob/master/third_party/terraform/website/docs/d/google_kms_crypto_key.html.markdown)

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This PR seems not to have generated downstream PRs before, as of ca52dcc.

Pull request statuses

No diff detected in terraform-google-conversion.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#845
depends: hashicorp/terraform-provider-google#3843

[danawillow]

Looks good, thanks @gracenoah!