Factor out valid_column_definition_options

Shopify is implementing a custom ActiveRecord adapter to integrate
with [Vitess](https://vitess.io/), and we would like to overload the
`valid_{column,table}_definition_options` methods and add
additional valid options for schema migrations.

For example:

```ruby
module ActiveRecord
  module ConnectionAdapters
    class VitessMysql2Adapter < Mysql2Adapter
      ...

      def valid_table_definition_options
        super + [:skip_vschema_migrations, :sharding_keys, :auto_increment]
      end

      def valid_column_definition_options
        super + [:skip_vschema_migrations, :sharding_keys, :auto_increment]
      end
    end
  end
end
```

This is the simplest possible change and factors out the various `valid_{table,column,primary_key}_definition_options` to be a public
method on an adapter instance.

activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb

@@ -586,7 +586,7 @@ def new_check_constraint_definition(expression, options) # :nodoc:
 
       private
         def valid_column_definition_options
-          ColumnDefinition::OPTION_NAMES
+          @conn.valid_column_definition_options
         end
 
         def create_column_definition(name, type, options)

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -1497,6 +1497,18 @@ def bulk_change_table(table_name, operations) # :nodoc:
         non_combinable_operations.each(&:call)
       end
 
+      def valid_table_definition_options
+        [:temporary, :if_not_exists, :options, :as, :comment, :charset, :collation]
+      end
+
+      def valid_column_definition_options
+        ColumnDefinition::OPTION_NAMES
+      end
+
+      def valid_primary_key_options
+        [:limit, :default, :precision]
+      end
+
       private
         def validate_change_column_null_argument!(value)
           unless value == true || value == false
@@ -1595,14 +1607,6 @@ def create_alter_table(name)
           AlterTable.new create_table_definition(name)
         end
 
-        def valid_table_definition_options
-          [:temporary, :if_not_exists, :options, :as, :comment, :charset, :collation]
-        end
-
-        def valid_primary_key_options
-          [:limit, :default, :precision]
-        end
-
         def validate_create_table_options!(options)
           unless options[:_skip_validate_options]
             options