relay: Add basic configuration and builder
This adds the basic TorRelayConfig object along a builder in order to create a TorRelay object.
At this commit, the configuration object only holds a "StorageConfig" which is a simple starting point which will allow to expand to a KeyMgr and then a ChanMgr along more configuration.
There is also no custom Error for the crate at this point.
Fixes #1534 (closed)
Signed-off-by: David Goulet dgoulet@torproject.org