Contributed metadata policies must be consistent with one another

[vdzhuvinov]

The Federation spec lays out 6 principles that govern the metadata policies:

https://openid.github.io/federation/main.html#name-principles

The spec and federations that adopt it will benefit from the clarification that the metadata policies that Trust Chains and Intermediates contribute to a Trust Chain must be consistent with one another. So that, if an Entity adds a policy that breaks the overall consistency, this must produce a policy error.

This principle is implied by the algorithms that define the operator combinations. Writing it down in the Principles section will help federations solve metadata policy disputes.

[vdzhuvinov]

PR: #183