Introduce common `messaging.consumer.group.name` and `messaging.destination.subscription.name` instead of per-system ones

[lmolkova]

Changes

Addresses consumer group and subscription name part of #797

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [x] Change log entry added, according to the guidelines in When to add a changelog entry.
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• [x] schema-next.yaml updated with changes to existing conventions.

[lmolkova]

It sounds like we have two popular terms - "consumer group" and "subscription" to describe load/balancing + fan-out. Some systems even use both notions (RocketMQ).

I'm totally open to other naming suggestions depending on the feedback.

[github-actions[bot]]

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