[Engine] Detailed addin load reporting

[Therzok]

ReportAddinLoad only reports the addin id which is being loaded.

To allow for finer grained control and automated verifying of load reasons, we should supply:
a) the load reason - loaded as dependency or loaded directly (i.e. extension objects)
b) the load phase - whether the addin is loaded just as metadata or fully loaded in memory

[Therzok]

cc @slluis

[mhutch]

It should also be possible to report when each assembly in an addin is loaded