daemon: refactor and modularize daemon and pkg/daemon

[rolbk]

This PR refactors the daemon codebase to prepare for standalone operation support. It separates concerns and moves reusable components to pkg/daemon.

This is part 1 of a series splitting #4853 into smaller, reviewable PRs. The original PR will be closed.

Main Changes

• Split daemon server: Separated the existing daemon gRPC server into transport (daemon/grpc/server.go) and core logic (DaemonEngine in pkg/daemon/private/engine) components. The engine contains the business logic, independent of transport.

• Move fetcher to pkg: Relocated daemon/fetcher to pkg/daemon/fetcher to make it accessible for in-process usage.

• Move drkey to private: Relocated daemon/drkey to private/drkey as it is shared infrastructure.

• Add ASInfo abstraction: Introduced pkg/daemon/asinfo to provide local AS information from topology files.

• Add trust utilities: Created pkg/daemon/private/trust with helpers for trust verification setup.

• Add AcceptAll verifier: Added private/segment/verifier/acceptall.go for scenarios where segment verification should be skipped.

• Rename daemon.go to connector.go: Clarified the purpose of the Connector interface in pkg/daemon.