Extract #build_change_column_definition

[adrianna-chang-shopify]

Summary

Define APIs on the MySQL and PostgreSQL connection adapters for building
ChangeColumnDefinition objects. These provide information on what a
column change would look like, when called with the same arguments as
would be passed to #change_column.

See #45625 for more context on exposing schema definition APIs.

Other Information

I've left out a definition for SQLite3, because it performs #change_column by replacing the entire table definition:

rails/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

Lines 311 to 318 in 74a9929

	def change_column(table_name, column_name, type, **options) # :nodoc:
	alter_table(table_name) do |definition|
	definition[column_name].instance_eval do
	self.type = aliased_types(type.to_s, type)
	self.options.merge!(options)
	end
	end
	end

We could define #build_change_column_definition to essentially wrap the new column definition, but this wouldn't actually be used by #change_column, and we wouldn't be able to set the ddl, so I opted to just ignore it.