Add transaction_name_groups config spec

[felixbarny]

For now, this option is only intended to be implemented in Java where we got a specific request. However, the option itself is not specific to Java and can also be implemented by other agent as the need arises. We should still make sure that the option is potentially useful and feasible to implement in other agents, too.

Some agents have a concept of filters or processors that let users achieve similar things. However, the intent of the option is to allow users to group transactions without requiring code changes.

Reference implementation: https://github.com/elastic/apm-agent-java/pull/2676

• [x] Create PR as draft
• [x] Approval by at least one other agent
• [x] Mark as Ready for Review (automatically requests reviews from all agents and PM via CODEOWNERS)
  • Remove PM from reviewers if impact on product is negligible
  • Remove agents from reviewers if the change is not relevant for them
• [ ] Merge after 2 business days passed without objections \ To auto-merge the PR, add /schedule YYYY-MM-DD to the PR description.

[apmmachine]

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#### Build stats * Start Time: 2022-07-04T04:10:00.694+0000 * Duration: 3 min 54 sec