Add support for squashing across AddIndex, AddConstraint, and AlterConstraint

[Markus Holtermann]

The AddIndex, AddConstraint, and AlterConstraint migration operations straight out reject squashing across them.

Let's take the following set of operations:

[
    migrations.CreateModel("A", fields=[("id", models.BigAutoField())]),
    migrations.AddIndex("b", models.Index(fields=["id"], name="b_idx")),
    migrations.AddField("a", "name", models.IntegerField()),
]

I would expect to get the following 2 operations:

[
    migrations.AddIndex("b", models.Index(fields=["id"], name="b_idx")),
    migrations.CreateModel("A", fields=[("id", models.BigAutoField()), ("name", models.IntegerField())]),
]

Instead, I end up with the same set of operations.

[Simon Charette]

I'm surprised to see that when we added IndexOperation and friends we didn't realize that AlterTogetherOptionOperation and friends were subclassing ModelOptionOperation for its .reduce operation (which make us of references_model to implement what you're after here).

This ticket seems to be about implementing model reference awareness but we could also implement field reference awareness (references_field) not that we have the machinery to extra field references from expressions (see Index.check for example).

[Simon Charette]

Left some comments on the MR.

We either need to drop the references_field optimization or implement it in a way that caters for all potential field references. The proposed changes disable the optimization in the presence of .expressions but it needs to do so when .include and .condition are present (or check them as well) and consider that .fields can contain descending order specifiers (e.g. Index(fields=['-pub_date'])).