add an action to create a bucket

### Context

when deploying minio to test other charms with s3, it'd be handy to have an action to create a bucket without having to manually connect to the client.

### What needs to get done

add an `add-bucket` action (and perhaps a corresponding delete-bucket?)

### Definition of Done

can do `juju run minio/0 add-bucket FOO` --> has a FOO bucket.