kms: add support for MinKMS and remove some unused/broken code (#19368)

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>

cmd/api-errors.go

@@ -2430,9 +2430,9 @@ func toAPIError(ctx context.Context, err error) APIError {
 		switch e := err.(type) {
 		case kms.Error:
 			apiErr = APIError{
-				Description:    e.Err.Error(),
 				Code:           e.APICode,
-				HTTPStatusCode: e.HTTPStatusCode,
+				Description:    e.Err,
+				HTTPStatusCode: e.Code,
 			}
 		case batchReplicationJobError:
 			apiErr = APIError{