add PKCE support to device code flow (#2575)

server/deviceflowhandlers.go

server/deviceflowhandlers_test.go

storage/conformance/conformance.go

storage/ent/client/devicetoken.go

storage/ent/client/types.go

storage/ent/db/devicetoken.go

storage/ent/db/devicetoken/devicetoken.go

storage/ent/db/devicetoken/where.go

storage/ent/db/devicetoken_create.go

storage/ent/db/devicetoken_update.go

storage/ent/db/migrate/schema.go

storage/ent/db/mutation.go

storage/ent/db/runtime.go

storage/ent/schema/devicetoken.go

storage/etcd/etcd.go

storage/etcd/types.go

storage/kubernetes/types.go

storage/sql/crud.go

storage/sql/migrate.go

storage/storage.go