ESNIP: Passthrough resolution

[jmacwhyte]

This ENSIP defines a standard by which a resolver may designate another resolver as a "passthrough" resolver. When the resolver receives a resolution query for which it holds no record, it forwards the query to the passthrough resolver and returns the result to the caller. Because each passthrough target may itself designate a further passthrough resolver, resolvers of increasing functionality can be daisy-chained together, with the newest resolver able to answer queries for all records set on any downstream resolver in the chain.

The standard resolution methods retain their existing signatures and forward silently, so the mechanism is invisible to existing ENS clients. Two new methods expose the passthrough resolver's address and query a resolver's own records without passthrough, making it possible to locate which resolver in a chain contains a given record.

Discussion: https://discuss.ens.domains/t/ensip-passthrough-resolution-enabling-painless-resolver-migration/22261

[github-actions]

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