Support "alter field" in automatic migration generator

[m4tx]

We should support changing metadata of existing fields in the automatic migration generator. In other words, if someone changes some properties about a database field without changing its name, we should be able to migrate this automatically.

The entry point for this is

cot/cot-cli/src/migration_generator.rs

Line 391 in 2bbb096

fn make_alter_field_operation(

. This will also require adding a new OperationInner variant:

cot/cot/src/db/migrations.rs

Line 460 in 2bbb096

enum OperationInner {

Note that the "alter field" operation will need the old version of the fields modified, too, so that an implementation of Operation::backwards can be created.