Dictionary coercion on min/max

[brancz]

Is your feature request related to a problem or challenge?

min/max doesn't support dictionary encoded arrays

Describe the solution you'd like

Just allow min/max tracked scalars to be coerced from dicts.

Describe alternatives you've considered

Casting my dict to flat, but bad idea for performance, that why I'm looking into this.

Additional context

I already have the code for this, so just opening this for tracking purposes.

@alamb