This commit adds support for MinKMS. Now, there are three KMS
implementations in `internal/kms`: Builtin, MinIO KES and MinIO KMS.
Adding another KMS integration required some cleanup. In particular:
- Various KMS APIs that haven't been and are not used have been
removed. A lot of the code was broken anyway.
- Metrics are now monitored by the `kms.KMS` itself. For basic
metrics this is simpler than collecting metrics for external
servers. In particular, each KES server returns its own metrics
and no cluster-level view.
- The builtin KMS now uses the same en/decryption implemented by
MinKMS and KES. It still supports decryption of the previous
ciphertext format. It's backwards compatible.
- Data encryption keys now include a master key version since MinKMS
supports multiple versions (~4 billion in total and 10000 concurrent)
per key name.
Signed-off-by: Andreas Auernhammer <github@aead.dev>
