Etcd storage backend

[gavincabbage]

Is your feature request related to a problem? Please describe.
Since my systems already use Etcd as a storage backend, an Etcd backend for Flipt would make it easier to deploy in my environment.

Describe the solution you'd like
I would like an Etcd storage backend, in addition to the PostgreSQL and SQLite options.

Additional context
Adding an Etcd backend will be a shift from the SQL backends currently offered, but hopefully any work towards Etcd would also be useful towards supporting other KV backends, like Redis.

[markphelps]

Let me know if I can help @gavincabbage !!

[stale]

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