Namespace server for AS2 extensions

[evanp]

One useful tool for creating context documents is a permanent URL server. https://purl.archive.org/ and https://w3id.org/ are two classics.

I could see some benefit in having a specific domain just for AS2 extensions, but the costs are pretty high. We'd need software for managing the URLs and an organization with the projected longevity to keep this service operating for the long term. (10 years? 100 years?)

Bringing this up here, but I think the existing services work fine, and we should probably use them at least for now.

[evanp]

This is related to #7