Support lease read

[xingyong]

What would you like to be added?

etcd raft has already supported two readonly options: ReadOnlySafe and ReadOnlyLeaseBased, and the default value is ReadOnlySafe. They can be configured in raft.Config. As we know, ReadOnlyLeaseBased is better for performance. So, etcd may be better to support this config in embed.NewConfig.

Why is this needed?

For performance considerations.