Integrate riverui queries into driver system, add Client.Schema()

[bgentry]

This integrates all of riverui's non-Pro queries into this library's driver system, which paves the way for riverui to support multiple driver backends (sqlite) and to add more Pro-specific UI extensions. These queries were generally imported as-is except for the sqlite variants which required some rework.

I also added a Client.Schema() accessor because I couldn't see any way for riverui to pull the schema from the river.Client in its constructor opts. Rather than add another param for it there, it seemed cleaner to just expose a reader method on the client. This all ends up being required if we want the UI to properly support custom schemas while also having it directly call into pilot and driver methods without having to expose all this functionality directly on the Client and bloating its API.