cli: poll messages as a consumer group, from a timestamp, and in follow mode #4446

Problem

iggy message poll reads one batch from one partition as a regular consumer.

• It always polls as a regular consumer (core/cli/src/commands/binary_message/poll_messages.rs:64). You cannot read a topic the way a group member reads it.
• It can start from --offset, --first, --last or --next (:55), but not from a timestamp. The SDK has PollingStrategy::timestamp (core/common/src/types/message/polling_strategy.rs:72).
• It reads one batch and exits. To watch a live partition, you must run it in a shell loop, for example with --next --auto-commit. Each run connects and logs in again.

Proposal

• Add -g, --consumer-group <GROUP>. The CLI joins the group and polls the partitions that the server assigns to it. On exit, it leaves the group, also after Ctrl-C or an error. -g conflicts with -c, --offset and <PARTITION_ID>.
• Add --timestamp <TIME>. Accept an RFC 3339 time, Unix microseconds, or a duration back from now (15min).
• Add --follow. Keep polling until Ctrl-C, and wait for --poll-interval (default 100ms) after an empty batch.
• Build the group and follow modes on IggyConsumer (core/sdk/src/clients/consumer.rs). It already joins the group, tracks the offset of each partition, commits, and leaves the group on shutdown. Turn off its automatic group creation.

Notes

• A member that disconnects without a leave keeps its partitions until consumer_group.session_timeout (default 30 s) expires.
• A join moves partitions from other members. A partition with uncommitted polled messages moves only after its owner commits them, or after consumer_group.rebalancing_timeout (default 30 s). So a single -g poll on a busy group often gets no partition. server: consumer group member that joins while a revocation is pending never gets a partition #4391 can also leave a new member without a partition.
• With --auto-commit, the group offset moves past the messages that the CLI prints, so the other members never get them. The help text must say this.
• Over HTTP, -g cannot work, because a group join returns FeatureUnavailable (core/sdk/src/http/consumer_groups.rs:116). Reject -g with --transport http before the CLI connects.
• The server has no long poll yet. When feat: add deferred polling timeout to PollMessages #3470 (deferred polling timeout) lands, use it in follow mode.