sdbus++: events: create json for events

Create JSON for the events containing their metadata and leverage that
as the "message" portion of the `sd_bus_error`. This will allow
unpacking the message

sdbus++: events: create json for events

Create JSON for the events containing their metadata and leverage that
as the "message" portion of the `sd_bus_error`. This will allow
unpacking the message on a client side back to the original exception.

Tested:
```
$ busctl --user call net.poettering.Calculator /net/poettering/calculator net.poettering.Calculator Divide xx 5 0
Call failed: {"net.poettering.Calculator.DivisionByZero":{"FOO":"unused"}}
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I4edd76d983267f28e51c4aa41902d45f5d6da793

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