Report secret version as a hash of the input and contents

[tomhjp]

Closes #146

• Generate a version based on the secret inputs and contents so that downstream consumers can tell the secret has been rotated
• Generate an error when duplicate object names are used so that we can confidently use object name as the key for versions we return - this was essentially a bug we didn't catch before because it results in overwriting files
• Create a higher level unit test with mocked HTTP servers so we can verify the version behaviour nearer the API level.

This PR also deletes some unused code (writeSecret() and validateFilePath()) along with their tests now that the provider no longer supports writing secrets itself. Happy to split that out into a separate PR if it makes the diff tricky to read.

[hashicorp-cla]

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