* Move storage clients to separate packages
* Allow setting of client credentials for AWS S3
* Use managed identity client secret or default credentials for Azure Blob Storage
* Use credentials file to authorize GCS client
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Signed-off-by: Daniel Weiße <dw@edgeless.systems>
* Replace external KMS backend logic for AWS, Azure, and GCP with go-kms-wrapping
* Move kms client setup config into its own package for easier parsing
* Update kms integration flag naming
* Error if nil storage is passed to external KMS
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Signed-off-by: Daniel Weiße <dw@edgeless.systems>