Add support for additional x-thorn header where context is provided

[jcsmorais]

As part of the refactor done in #212 a decision was made towards having x-thorn header matching the username of the agent triggering a certain action, when this action is triggered by Fixtures, making it behave pretty much the same way like everywhere else when Agents trigger their own actions.

In order to provide additional information while debugging we want to introduce an additional x-thorn header where context is provided, e.g: X-Thorn-Context: Fixtures or X-Thorn-Context: MetadataFetcher

[rhoggSugarcrm]

We should probably also update THORN_VERBOSE to include this new information as appropriate.