[META 604] Implement `trace_continuation_strategy `

[![issue details](https://img.shields.io/endpoint?label=meta-issue:&url=https%3A%2F%2Fgiss.app.elstc.co%2Fapi%2Fstatus%2Felastic%2Fapm%2F604)](https://github.com/elastic/apm/issues/604)

Spec issue: https://github.com/elastic/apm/issues/596