finish the package maintainerships workflows

Two api endpoints to implement here, at least:

- `/packages/authored-by/:namespace` : packages *owned* by the namespace ("namespace" is in their name0
- `/packages/maintained-by/:namespace`: packages that the namespace has write permissions for (not necessarily in that namespace). Now that I think about it, shouldn't that be packages the user can write? Users are part of namespaces, and users have maintainership relationships with packages, so... (@chrisdickinson confirm intentions here?)

Then implement cli commands to invoke these endpoints.