consistency of terminology between 'provider' and 'publisher'

[Will]

In the filecoin world, 'storage provider' is used to refer to sources of content, which will be some of the entities who act in this 'index provider' role.

In engine/, this code base uses the term publisher to describe the role of exposing the chain of advertisements, and provider (found in schema.Advertisement) to describe the actual source of data from a client's perspective.

We should see if there's either a 3rd term we can use, or another pattern to help make this less confusing, because it is non-intuitive that running an index provider is actually running a publisher and not a provider.

[Andrew Gillis]

Perhaps replace publisher with index provider, and provider with content provider?