Extend Magpie resource ID by external ID extension

[fmigneault]

The following link to the referenced Magpie resource is provided:

stac-populator/STACpopulator/extensions/thredds.py

Lines 161 to 166 in c995b7e

	@property
	def links(self) -> list[pystac.Link]:
	"""Return a link for this resource."""
	url = self.access_urls[ServiceType.httpserver]
	link = magpie_resource_link(url)
	return [link]

To help distinguish it from any other links, the following could be used:
https://github.com/stac-extensions/external-ids

Similar references could be defined for "marble" (https://github.com/DACCS-Climate/marble-stac-extension) related IDs between nodes and their respective STAC Item definitions (notably if combined with Magpie Network feature).