We stopped using the topology.WithConnString option after introducing ApplyURI but this means that the topology doesn't know the URI used to create the Client and therefore cannot actually do things like SRV polling for mongos discovery.

I propose the following:

1. Store the initial URI in ClientOptions and add a ClientOptions.GetURI method. If ApplyURI was never called during construction, this returns the empty string.
2. Add a topology.WithURI option that takes the raw non-parsed URI as a string. This allows the topology to figure out if an SRV URI was used and perform SRV polling if necessary.