provide uri-formatted information on all connections

[arlimus]

Since Train provides Train.target_config( ... ) which resolves target URIs to their transports, this PR provides the a mechanism to reverse the direction and go from Connection to URI.

[arlimus]

The reason this doesn't use to_s is that this function currently offers all options configured with a Connection in its output. This continues to be very helpful and shouldn't change. At the same time it is too much information for a simple URI pattern. Finally, it can potentially expose passwords and other secrets which this MR keeps to a minimum (i.e. don't show passwords or keys).

[chris-rock]

Very useful addition @arlimus