Roundabout support for Forge network

[alanshaw]

The Forge network requires UCAN authorized retrievals, but roundabout simply redirects to a URL. Roundabout needs to obtain a delegation on behalf of the network in order for the Spade client to download segments of an aggregate from the Forge network.

[alanshaw]

Per storacha/RFC#79 we will no longer utilize roundabout. That functionality will be moved into Spade. See storacha/spade#12