Allow modules to provide flexible template row styles

Description of the need

Flexible templates have predetermined set of row styles provided by the function layout_flexible_row_styles(). There is a respectable selection there but if a user wanted a row with more that four columns, this is not provided; neither is there any way to provide alternate column widths.

We need a hook to allow modules to offer additional styles.

Proposed solution

A new hook_layout_flexible_row_styles() and hook_layout_flexible_row_styles_alter()

Alternatives that have been considered

None