[matrix] Provide possibility of using encryption on notify.notify_to_matrix

matrix provides end-to-end encryption. It would be nice to be able to use that to secure the communications.

Expected:
When configuring the matrix notify, say encryption : True

Have some sort of workflow to exchange keys (either TOFU trust on first use, or when creating the initial room “manually”, enable encryption, then export the keys and put them in the configuration file of hass.

Home Assistant release (hass --version):
0.60.0

Python release (python3 --version):
Python 3.4.2

Component/platform:
notify.notify_to_matrix