Implement signing mechanism for families of providers

[willscott]

We see cases where providers:

• want to provide different data protocols at different endpoints
• want an alias at a new node with a new identity

we should be able to handle these cases efficiently. To do so we need:

• signing from both identities
• message indicating the additional identity being added / removed

[willscott]

A design is at #804

This issue should now track implementation of the spec

[momack2]

Is work happening against this somewhere? AFAIK this is going to be completed and launched by EOQ - correct?

[willscott]

yes
@ischasny has implemented this - it's in v0.9.2 of the provider with #292
and here in #1014 - which is rolled out to the cid.contact indexers.