deferred - Implement task parameter promotion to headers

To implement killswitches in the future, we need methods on `TaskNamespace` that allow parameters to be promoted into task headers

`task_ns.promote_to_header(parameter_name, tasks=['foobar'])`

is the method signature/example from the [design doc](https://www.notion.so/sentry/Worker-Tasks-Topic-Worker-74a84c8b1992480190c380e282f87778?pvs=4#b34bae011f3e4b4f818387cda0b60b0e)