Support grouped Unix endpoint addresses

[samuel-williams-shopify]

Summary

• Accept IP addresses and Unix-domain-socket paths when building EDS endpoints.
• Keep alternative addresses for one logical backend grouped using Envoy additional_addresses.
• Allow generated clusters to select HTTP/1 or HTTP/2 upstream protocol behavior.
• Reject endpoint records without any addresses.

Used by socketry/async-service-supervisor-envoy#1 to publish the bound Falcon listeners from socketry/falcon#356 as Envoy upstream endpoints.

Testing

• bundle exec bake test (66 tests, 140 assertions)
• bundle exec rubocop