[query/ggplot] Adds configurable legend format to point geom

[iris-garden]

This change makes the labels for groups of points on scatterplots created through geom_point configurable by the user.

Consider the following sample code:

import hail as hl
from hail.ggplot import ggplot, aes, geom_point
ht = hl.utils.range_table(10)
ht = ht.annotate(squared=ht.idx ** 2)
ht = ht.annotate(even=hl.if_else(ht.idx % 2 == 0, "yes", "no"))
ht = ht.annotate(threeven=hl.if_else(ht.idx % 3 == 0, "good", "bad"))
fig = (
    ggplot(ht, aes(x=ht.idx, y=ht.squared, color=ht.even, shape=ht.threeven))
    + geom_point()
)
fig.show()

Before this change, this code generates the following legend for the plot:

After this change, the legend looks like this (× is now the default group name separator):

If legend_format is passed to geom_point, like so:

...
    + geom_point(legend_format="{color} ({shape})")
...

the following legend will be generated, replacing the names of the aesthetics within brackets in the format string with their values:

Notably, if the reverse ordering of group names ("{shape} ({color})") is used, the legend will look like this:

The order in which the groups in the legend appear can be adjusted, if desired, by changing the order of the aesthetics when they are passed into aes:

...
    ggplot(ht, aes(x=ht.idx, y=ht.squared, shape=ht.threeven, color=ht.even))
    + geom_point(legend_format="{shape} ({color})")
...

producing the following legend: