Allow extensions to contribute a command to viewContainer title bar

[ejizba]

Rather than contribute a command at the "view" level which is possible today, we want to contribute a command at the "viewContainer" level. In our scenario, we're trying to clean up duplicate subscription nodes in the Azure views:

I would love to have a dropdown that shows the current subscription, but would also settle for a simple button:

It seems like other viewContainers not contributed by extensions already support this, like Debug and Remote:

cc @fiveisprime

[miguelsolorio]

We also have #89243 regarding using the dropdown in the sidebar header

[gjsjohnmurray]

#110218 would combine well with this, allowing a contributed command to produce a submenu of your subscriptions. So you may like to upvote it.

[gplumb]

How can we go about re-opening this issue?

[gjsjohnmurray]

@gplumb if #108271 makes some progress perhaps it will get generalized beyond just the Explorer.

cc @eamodio @isidorn