Add :skip_generating_fields option of the table method

[andrykonchin]

The :skip_generating_fields option prevents implicit declaration of some default fields with specified names (e.g. :id, :created_at, :updated_at) so they can be declared manually without warnings later:

table skip_generating_fields: [:created_at, :updated_at]

field :created_at, :datetime, store_as_string: true
field :updated_at, :datetime, store_as_string: true

Close #924

[github-actions]

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