proposal: x/crypto/ssh/knownhosts: create from ...io.Reader

`knownhosts.New` accept file on local disk only now, which is not flexible for parsing knowhosts inside memory or load from other media.

Proposal  for new API

```
func NewFromReader(reader ...io.Reader) (ssh.HostKeyCallback, error)
```

to load knowhosts from ` io.Reader` s