les: handler separation (#19639)

core/blockchain.go

les/api.go

les/api_backend.go

les/api_test.go

les/benchmark.go

les/bloombits.go

les/client.go

les/client_handler.go

les/commons.go

les/costtracker.go

les/distributor.go

les/distributor_test.go

les/fetcher.go

les/fetcher_test.go

les/handler_test.go

les/metrics.go

les/odr.go

les/odr_test.go

les/peer.go

les/peer_test.go