Expose `endpoint`

[mixmix]

In the CLI we want to be able to access the endpoint used to instantiate entropy with for logging reasons.
Right now it's not super accessible (it might be buried under entropy.substrate somewhere but I couldn't get to it easily)

Recommend new getter entropy.endpoint

[rh0delta]

The endpoint is always accessible if we add the endpoint option to each of the commands. Endpoint will be accessible through opts.endpoint