Implement transaction_name_groups config

The Node.js agent supports the [`usePathAsTransactionName`](https://www.elastic.co/guide/en/apm/agent/nodejs/current/configuration.html#use-path-as-transaction-name) option.

A risk of using this option is that it can lead to cardinality issues when the path contains parameters. To mitigate this, the Node.js agent should
- Implement the [`transaction_name_groups `](https://github.com/elastic/apm/blob/1f9eeee05c9bdb46819e540be4c99b6a55f8a341/specs/agents/tracing-transaction-grouping.md) option
- Add a warning to the [`usePathAsTransactionName`](https://www.elastic.co/guide/en/apm/agent/nodejs/current/configuration.html#use-path-as-transaction-name) docs, similar to the one from the [Java agent](https://www.elastic.co/guide/en/apm/agent/java/current/config-http.html#config-use-path-as-transaction-name).