Introduce new command bar design behind feature flag

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This introduces a new command bar component, based off Fluent UI 9, behind a feature flag. It also moves the command bar under the tab bar, as specified in the new designs.

To activate the new command bar, you need the feature.commandBarV2=true feature flag (which is enabled in the preview link above). I wanted to do a bit more iteration here before pushing this live, and I'm also interested in figuring out if/how we can control this flag in production to have even more flexibility here. The design spec also includes changes to the button layout and groupings of buttons which is not yet present, but will be done behind the feature flag in later PRs.

This PR does include a few small changes that will affect the old command bar too:

1. Refactored some of the logic in CommandBarComponentAdapter into common methods (specifically the logic around Postgres/Mongo shell buttons)
2. Changed the onCommandClick handler for command buttons to take an Explorer, thus allowing all the button creation logic to run without having an Explorer. This allows buttons to be created as static constants, where appropriate, or in other environments that don't have access to Explorer, while still allowing them to take actions on the Explorer when clicked. The only thing we were using Explorer for when constructing the buttons was to capture it for the onCommandClick handler, so this seemed like a safe refactor. The Explorer is provided when we translate our "abstract" CommandButtonProps into an actual button for rendering.

A quick preview of the new Command Bar:

Because of it's location, there are some weird things. The global account-level "Call to Action" buttons (Synapse, Data-Plane RBAC, etc.) are in the command bar under the tab bar. Other global account-level actions (New Container/Database) were moved to the sidebar, but we haven't figured out the right home for the "Call to Action" buttons, DE settings, portal full screen, etc.

It works for now, but we're continuing to iterate on that. For example, if you have no tabs open, the command bar is still present (in order to show the global account-level actions).

Another quirk of the new command bar for now, is that the Fluent UI 9 toolbar doesn't have a great Split Button/Menu experience. You can put a standard split-button in, but it looks clunky. For now, I've used a standard button for commands with children:

I'm also working on improving this.

Since this is behind a feature flag, we can continue iterating on these before we start to roll it out to customers.