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>

internal/kms/dek_test.go

@@ -41,6 +41,13 @@ var dekEncodeDecodeTests = []struct {
 			Ciphertext: mustDecodeB64("eyJhZWFkIjoiQUVTLTI1Ni1HQ00tSE1BQy1TSEEtMjU2IiwiaXYiOiJ3NmhLUFVNZXVtejZ5UlVZL29pTFVBPT0iLCJub25jZSI6IktMSEU3UE1jRGo2N2UweHkiLCJieXRlcyI6Ik1wUkhjQWJaTzZ1Sm5lUGJGcnpKTkxZOG9pdkxwTmlUcTNLZ0hWdWNGYkR2Y0RlbEh1c1lYT29zblJWVTZoSXIifQ=="),
 		},
 	},
+	{
+		Key: DEK{
+			Version:    3,
+			Plaintext:  mustDecodeB64("GM2UvLXp/X8lzqq0mibFC0LayDCGlmTHQhYLj7qAy7Q="),
+			Ciphertext: mustDecodeB64("eyJhZWFkIjoiQUVTLTI1Ni1HQ00tSE1BQy1TSEEtMjU2IiwiaXYiOiJ3NmhLUFVNZXVtejZ5UlVZL29pTFVBPT0iLCJub25jZSI6IktMSEU3UE1jRGo2N2UweHkiLCJieXRlcyI6Ik1wUkhjQWJaTzZ1Sm5lUGJGcnpKTkxZOG9pdkxwTmlUcTNLZ0hWdWNGYkR2Y0RlbEh1c1lYT29zblJWVTZoSXIifQ=="),
+		},
+	},
 }
 
 func TestEncodeDecodeDEK(t *testing.T) {