This commit adds support for the minio/kes KMS.
See: https://github.com/minio/kes
In particular you can configure it as KMS by:
- `export MINIO_KMS_KES_ENDPOINT=` // Server URL
- `export MINIO_KMS_KES_KEY_FILE=` // TLS client private key
- `export MINIO_KMS_KES_CERT_FILE=` // TLS client certificate
- `export MINIO_KMS_KES_CA_PATH=` // Root CAs issuing server cert
- `export MINIO_KMS_KES_KEY_NAME=` // The name of the (default)
master key
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