Support layer style copy from special layer_styles table

[eblondel]

Layer styles can be managed in a standardized Postgres table, and managed through QGIS https://www.cadlinecommunity.co.uk/hc/en-us/articles/360018377598-QGIS-Storing-Styles-in-Databases

The objective here is to handle the detection of this layer_styles table in case of DBI handlers (entity_handler_dbi_df, entity_handler_dbi_df_geometry_columns). If the table is available, we look for styles associated with the target entity db table. If there are any style, we list them as entity data styles, and attach their definitions (subset of layer_styles table) as entity resource so they can be be copied as XML through a dedicated entity$copyStylesToDataDir, once entity job dir is created (at execution level).

@wheintz