Support registering views to the secondary side bar

[benibenj]

Copilot Generated Description: Introduce support for contributing views to the auxiliary bar, allowing extensions to register custom view containers in the secondary side bar. This includes updates to the views extension handler and the addition of a placeholder for the auxiliary bar contribution point.

This was inspired by #156526

closes #151681

[bpasero]

Not seeing any red flags, but I think @sandy081 should give the final 👍

I wonder if we should leak the technical term "auxiliary bar" into the API, why not rather just secondary bar, or secondary side bar.

[benibenj]

That's a good point. I'm not to sure what we should use as the primary side bar is called activityBar and so in theory this would be called secondaryActivityBar but that sounds weird. I changed it to secondarySideBar which seems reasonable to me.