[BUG] Layer Attribution Option

## Context

What results were you expecting?

Some external layers require attribution as part of their licensing and terms of use. Many mapping libraries implement this as an `attribution` key on a layer definition that becomes a rendered over the map static text (sometimes HTML is supported).

BaseMap Layers might already have this specified in their style json, but will likely come up with many layer types particularly BitMapTileLayer where an external XYZ tile service is used.
https://maplibre.org/maplibre-style-spec/sources/#attribution
https://maplibre.org/maplibre-gl-js/docs/API/classes/AttributionControl/

This may not be a critical feature for users viewing their own notebooks, but will be an issue as they shared rendered notebooks or export images.

Start with the MapLibre source attribution and AttributionControl documentation linked in the issue, then trace how layer definitions and exported images are rendered in Lonboard. Identify how attribution should be represented for external layers such as BitmapTileLayer and shared notebooks. Done means required attribution is visibly rendered for applicable layers and remains present in shared or exported output.

Written by the indexing model from the issue text.