Specify the IPNI federation protocol

Specify the initial IPNI federation protocol which aims to achieve
eventually consistent index records across a collaborating set of nodes.

The federation protocol consists of four fundamental steps:
Initialization, Periodic snapshot taking, Exchange of snapshots and
Reconciliation. The protocol takes advantage of the immutability of
advertisements exposed by each provider to resolve conflicts across
indexers.

The specification lists a set of APIs exposed by a participating indexer
in order to enable the implementation of the federation protocol.