text: Title: Rename functions in aggregated_transactions helper
  Body: Some functions in the helper include `ForAggregatedTransactions` in the name but they are meant to be used for both single transactions and aggregated transactions.
  
  The term `aggregated` can be removed from such methods. (e.g. https://github.com/elastic/kibana/blob/a2ac439f56313b7a3fc4708f54a4deebf2615136/x-pack/plugins/apm/server/lib/helpers/aggregated_transactions/index.ts#L81).
  
  In addition, they could be moved to another helper.