OpenID Connect Federation 1.0: Refactor OIDC Federation metadata policy handling (after interop 3)

[Yavor Vasilev]

MetadataPolicyEntry:

• New PolicyEntryValidator interface, with default implementation
• Validate ops in constructor - throw policy validation exception if invalid
• Update constructor and parse method to use PolicyEntryValidator
• When applying the policy to a metadata parameter, observe new ordering of operations - mutation first, checks second.

Rename classes for new terminology, where necessary.

Consider adding support for getting the final RP metadata after resolved policy is applied.

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[Yavor Vasilev]

Check whether task still applicable when draft 27 gets published.