Accept listener as positional argument

[samuel-williams-shopify]

Align the Envoy supervision hook with Falcon::Environment::Server#prepare_worker!, where both the instance and listener are required lifecycle objects.

This supports socketry/falcon#356.

Verification:

• bundle exec bake test (26 tests, 58 assertions)
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib (35/35)