Add ability to track agent separately from instrument in assertions

[AleitaT]

Currently, assertion object information is limited and lacks complete context to tell from where and from who the assertion was generated. This has largely been due to the nature of the information being very nearby or in the same system, so we knew where it came from. As assertion processing and environments expand and grow, it makes sense to provide more information to enable other systems to use this data to tell both a) how it was created (what system, tool, etc) and b) whom (system, or person) generated this assertion.

For example:
Kristen asserted X about Y using instrument VLRC
Moodle asserted X about Y from instrument MoodleAssessment
Salesforce asserted X about Y from instrument AssertionBot

https://schema.org/instrument
schema:instrument

[Lomilar]

Instrument can be harvested from the evidence (if it came in via xAPI, the xAPI statement may somehow tell the system where it came from) but this is more explicit. I don't know how the instrument would be used downstream, but +1.