enterprise(dev): allow custom generation key and set test key
Created by: bobheadxi
https://github.com/sourcegraph/dev-private/pull/14
ran into issues while working on https://github.com/sourcegraph/sourcegraph/pull/11008 , more context: https://sourcegraph.slack.com/archives/C07KZF47K/p1590596893006400
- automatically export the
dev-private
key as the signing key in dev - only use hardcoded public key if no private key is provided