Support "remove field" in automatic migration generator

[m4tx]

We should support removing fields in the automatic migration generator.

The entry point for this is

cot/cot-cli/src/migration_generator.rs

Lines 404 to 411 in 2bbb096

	#[must_use]
	fn make_remove_field_operation(
	&self,
	_migration_model: &ModelInSource,
	_migration_field: &Field,
	) -> DynOperation {
	todo!()
	}

. This will also require adding a new OperationInner variant:

cot/cot/src/db/migrations.rs

Line 460 in 2bbb096

enum OperationInner {

Note that the "removed field" operation will need to store all the data about the removed field, too, so that an implementation of Operation::backwards can be created.

[Soroushsrd]

if you're satisfied with #221 i shall continue with this issue

[m4tx]

Yeah, #221 looks good, feel free to continue with this one!