examples: add OGC services browser example

[Desplandis]

Description

This PR add an interactive example for discovering and loading OGC services through @camptocamp/ogc-client on a globe.

Motivation and Context

We already provides examples using WMSSource, WMTSSource, and WFSSource, but those relies mostly on hand-written source configs. This severely limits users' ability to easily test and visualise the rendering of their own services.

This new example aims to close that gap. Users can paste a service URL, browse the available layers and add them directly to a globe view.

Known limitations include:

• Private endpoints are not supported yet. I have a branch which adds basic auth support but requires to override the texture fetching stage.
• WMTS tile matrices currently assumes that the matrix identifier is a zoom-level integer. Addressing this properly would require support for tile grids within iTowns.
• Little to no styling of UI components