Inconsistent naming get_json() in EventGridEvent and EventGridOutputEvent

Both mentioned classes have methods called `get_json` which return the classes `__data` field. This seems odd for two reasons:

1. They don't actually return JSON but a dict (if the typing hints in `__init__` are correct). In any case this is a major source of confusion.
2. All other getters are implemented as properties. (So why not just do the same here?)

https://github.com/Azure/azure-functions-python-library/blob/44712a77ce6ba68065d1c16ed6015bb998ddfe48/azure/functions/_eventgrid.py#L33-L34

https://github.com/Azure/azure-functions-python-library/blob/44712a77ce6ba68065d1c16ed6015bb998ddfe48/azure/functions/_eventgrid.py#L86-L87

Start in azure/functions/_eventgrid.py at lines 33-34 and 86-87, then compare these accessors with the other getters on EventGridEvent and EventGridOutputEvent. Resolve the naming and property behavior consistently, while confirming that the resulting API reflects whether the value is a dict rather than JSON.

Written by the indexing model from the issue text.