Feature: if_exists option for remove_column method in migrations

It’s rather convenient using if_exists option for drop_table method in migrations. Unfortunately, this option is not allowed for remove_column, but can be implemented for PostgreSQL adapter. SQlite3 allows removing column twice without complaints. MySQL would still raise an error on second attempt to delete the same column as it doesn’t support this option.

This PR adds possibility to pass if_exists option to remove_column method in migrations for PostgreSQL. So that it’s possible to write a migration deletes a column and its down step doesn’t matter:

class RemoveUserName < ActiveRecord::Migration[5.2  ]
def change
    remove_column :users, :name, nil, if_exists: true
  end
end

``

If you support this change, I would appreciate your vote (and approval or feedback from maintainers) in this pr: https://github.com/rails/rails/pull/34566