This commit introduces a new package `pkg/kms`.
It contains basic types and functions to interact
with various KMS implementations.
This commit also moves KMS-related code from `cmd/crypto`
to `pkg/kms`. Now, it is possible to implement a KMS-based
config data encryption in the `pkg/config` package.
This commit adds a new admin API for creating master keys.
An admin client can send a POST request to:
```
/minio/admin/v3/kms/key/create?key-id=<keyID>
```
The name / ID of the new key is specified as request
query parameter `key-id=<ID>`.
Creating new master keys requires KES - it does not work with
the native Vault KMS (deprecated) nor with a static master key
(deprecated).
Further, this commit removes the `UpdateKey` method from the `KMS`
interface. This method is not needed and not used anymore.
