Provide configuration to group http-request transactions

`http-request` transactions currently capture the method and url as the transaction name, however the url could include unique ids which would result in many transaction groups in the APM UI. We can also potentially provide a grouping mechanism for all transactions on the agent side, e.g. using regex.

More detail: https://discuss.elastic.co/t/elastic-apm-rum-js-agent-grouping-http-request-transactions/215380

Start with the current `http-request` transaction naming behavior described in the issue and review the linked Elastic discussion for the proposed grouping requirements. Done means providing a documented configuration that groups requests with unique URL IDs into useful APM transaction groups, with the grouping behavior agreed and verified.

Written by the indexing model from the issue text.