* Get concrete types out of dynamic payload

wip

pull out value setting to func

* Add TestEventSTream_SetPayloadValue

Add more assertions

use alias type in unmarshalJSON to handle payload rawmessage

shorten unmarshal and remove anonymous wrap struct

* use map structure and helper functions to return concrete types

* ensure times are properly handled

* update test name

* put all decode logic in a single function
Co-authored-by: Kris Hicks <khicks@hashicorp.com>