Add drop NOT NULL operation to remove NOT NULL from a column

[andrew-farries]

Add a new sub-operation to the 'alter column' operation to drop NOT NULL constraints from columns.

Currently, it is only possible to set NOT NULL constraints on columns but not remove them. The syntax for doing this is:

{
  "name": "16_set_nullable",
  "operations": [
    {
      "alter_column": {
        "table": "reviews",
        "column": "review",
        "nullable": false,
        "up": "(SELECT CASE WHEN review IS NULL THEN product || ' is good' ELSE review END)",
        "down": "review"
      }
    }
  ]
}

Setting nullable: true in the above migration would result in a validation error saying that removing NOT NULL constraints was not supported. This PR removes this restriction and allows nullable: true to remove an existing NOT NULL constraint.

A migration to remove a NOT NULL constraint looks like:

{
  "name": "31_unset_not_null",
  "operations": [
    {
      "alter_column": {
        "table": "posts",
        "column": "title",
        "nullable": true,
        "up": "title",
        "down": "(SELECT CASE WHEN title IS NULL THEN 'placeholder title' ELSE title END)"
      }
    }
  ]
}

The differences between adding and removing a NOT NULL constraint are:

• nullable: true vs nullable: false
• The roles of up and down SQL are reversed; down now needs to rewrite any NULLs to meet the NOT NULL constraint on the old column. up defaults to a simple copy of the data from the old column to the new.

Fixes #223