feat: url attribute for connect.Client

[joshyam-k]

It would be a nice convenience to add an attribute to connect.Client that lets you quickly retrieve the server url.

You can currently pull this in from the environment in most cases, but it would be simpler to be able to pull it directly from here.

[tdstein]

@joshyam-k do you want to the API URL or the base server URL: https://connect.example.com/ or https://connect.example.com/__api__?

[joshyam-k]

was thinking base server URL for this.