Add support for deleting an enterprise organization

[tjcorr]

With the new addition of a rest API to delete organizations within a GitHub Enterprise (see changelog and go sdk) this PR adds support to delete a github_enterprise_organization.

Resolves #872

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Behavior

Before the change?

Previously organizations had be deleted through the GitHub UI and then remove from the state file manually using terraform state rm.

After the change?

Organizations can be removed with a terraform destroy.

Other information

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Additional info

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Added the appropriate label for the given change

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes (Please add the Type: Breaking change label)
• No

If Yes, what's the impact:

• N/A

Pull request type

Please add the corresponding label for change this PR introduces:

• Bugfix: Type: Bug
• Feature/model/API additions: Type: Feature
• Updates to docs or samples: Type: Documentation
• Dependencies/code cleanup: Type: Maintenance

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[tjcorr]

Note: I did write a test that exercises the full lifecycle of an enterprise organization and ran it successfully locally. I don't believe the CI jobs test out the enterprise functionality though. This might require @kfcampbell or another maintainer to update the PAT used in the tests to have enterprise admin permissions. I wasn't sure how you would want to handle that.

[kfcampbell]

Integration tests are passing for me locally as well. You are correct that integration tests are not exercised on CI, which is obnoxious and unfortunate. #1414 contains a little bit more information about this situation.