Add ggplot2 scales for agg_key structure

[mitchelloharawild]

Should correctly format <aggregated> in facets.

[mitchelloharawild]

As far as I can tell, this needs to wait for ggplot2/scales to use vctrs more.

In particular,

• scales::train_discrete() needs to make the discrete check extensible.
• ggplot2::ScaleDiscrete$get_breaks() needs to use vctrs set_intersect() (set_intersect(), set_union(), set_diff(), and set_equal() r-lib/vctrs#548)

[mitchelloharawild]

An alternate solution (likely required for any timely release) is to write ScaleAggDiscrete for aggregated discrete scales. Largely inherit from ggplot2::ScaleDiscrete.

[mitchelloharawild]

Consolidated into #192