SceneAppPage: Page background option

[torkelo]

Problem)

There has been a long-standing inconsistency in page designs / main background. Between pages that use the canvas background (Dashboard, Explore, Drilldown) and all other pages that use the primary background.

Most or all of the scene apps on cloud (o11y, frontend o11y, kubernetes app, and others) use primary background but look very much like dashboards (but with tabs). For more on this specific problem, there is this doc I wrote a long time ago

I have tried many many times to solve this inconcisteny over past 3 years.

• Switching to canvas as the default background for all pages (Works great in dark theme, but a gray surface does not look ideal in light theme).
• Switching to primary as the dashboard background (ie only borders as a separator around panels)
• Any many permutations of the above.

But none of my attempts to solve it have looked great. Before summer I stumbled on this solution of embracing this inconsistency rather than trying to get rid of it. So this it was this PR proposes.

This adds a new background option to SceneAppPage and SceneApp (can set a default for the whole app). OSS PR: grafana/grafana#111174