Support for etcd

[Jiawei0227]

What would you like to be added

Support for etcd key value storage

Why this is needed

we need to deploy smallstep on k8s based envs and kubernetes already have etcd storage so we should not need additional key value DB in that case

[hslatman]

Hi @Jiawei0227, we've discussed adding etcd to the supported types of storages, and agree that it could be useful.

However, we'll have to wait for #53 to be completed before it can be added, as it would likely require refactoring otherwise. With #53 merged it becomes possibly to maintain "storage drivers" outside of this repository, which would make it possible to support etcd too.