Capacity to configure SQL layers, either directly (data sql) or through sql file

[eblondel]

The configuration of SQL layers could be done in two ways:

• as SQL file path / URL put to a datasource. In that way geoflow will try to read the SQL file to pick up the SQL query (then set using geoflow_data$setSql()), or
• directly as sql data property in the Data column

Some code will be experimented to read the SQL query, if a Db driver is set-up in the configuration. Reading the SQL query will enable dynamic metadata retrieval, as it is already done for shapefile (uploadType = 'shp')