stack: add 'docker stack logs' command

[dlarochette]

What I did

Added a docker stack logs subcommand that fetches the logs of all services in a stack and aggregates them into a single stream, with each line prefixed by the (colored) name of the service it belongs to — the swarm equivalent of docker compose logs.

How I did it

• ServiceList filtered on the com.docker.stack.namespace label (same filter as the other stack subcommands), erroring with the usual nothing found in stack: message when empty.
• One goroutine per service calling ServiceLogs, demultiplexing the stream with stdcopy.StdCopy into a small line-buffered prefix writer. A shared mutex keeps concurrent lines intact; partial writes are buffered until a newline so lines are never split.
• Rotating ANSI color palette per service (same spirit as compose), --no-color to opt out. Prefixes are padded to the longest service name so output aligns.
• Flags supported: -f/--follow, --since, -n/--tail (per service), -t/--timestamps, --no-color — all mapping directly to the service logs API.

How to verify it

Unit tests cover argument validation, API error propagation, option passthrough, output prefixing, and the line-buffering writer:

go test ./cli/command/stack/ -run 'TestStackLogs|TestPrefixWriter|TestLogPrefix' -v

Verified manually on a 2-node swarm (services on both manager and worker):

$ docker stack logs --tail 2 --no-color d2
d2_local  | from-manager
d2_local  | from-manager
d2_remote | from-worker
d2_remote | from-worker

--follow streams continuously; --timestamps and --since behave as for docker service logs.

Description for the changelog

Add `docker stack logs` command to fetch aggregated, per-service-prefixed logs of all services in a swarm stack.