`RSConnectExecutor.server_settings` throws error if `self.client` is a `PositClient` object

[wch]

With a RSConnectExecutor object, if .server_settings is accessed and self.client is a PositClient object (as opposed to a RSConnectClient object), it will throw an error.

I'm running into this problem while working on #545, when I try to deploy a Shiny app to shinyapps.io.

The problem is that the server_settings property accesses self.client.server_settings. self.client can be a RSConnectClient or PositClient object. However, the PositClient class does not have .server_settings.

https://github.com/rstudio/rsconnect-python/blob/34c77ea280c353ebdc8f2987f31765736aa6f96f/rsconnect/api.py#L916-L919

Maybe the PositClient class could have a .server_settings field, set to None or an empty dictionary?