Allow adding attributions when specifying a map layer in the app-config TOML file

[jahow]

Description

I would like to be able to specify an attributions text for a custom layer when configuring the Datahub. This field is not supported yet in the app config, see: https://geonetwork.github.io/geonetwork-ui/main/docs/guide/configure.html#map

Motivation

Attributions are essential when showing a map to a user

Inspiration and context

This is supported by the map context system in gn-ui, see for example the default basemap which has attributions built-in: #890