[go-experimental] Allow overriding server URL

Created by: thiagoarrais

Is your feature request related to a problem? Please describe.

Since #4755 the go-experimental generator supports selecting servers, but I can't see how to override server URLs with an arbitrary basepaths. That would be useful, for example, when testing a client generated with production spec against an in-development version of a server.

Describe the solution you'd like

Having a context option for setting an arbitrary basepath would be nice. This maintains the spirit of #4755 of configuring the client via context options.

It would be configured like so:

ctx := context.WithValue(context.Background(), sw.ContextBasepath, "http://localhost:8000/in-development")