add support for multiple smtp recipient addresses

[buzz0r]

This is especially useful for dual cameras like the Reolink Duo which supports to send smtp with more than one recipient address to trigger motion on both rtsp streams. The Reolink Duo does not support two different smtp setups for the two build-in cameras, so this is the only working solution to get motion detection triggered for both rtsp feeds.

[seydx]

Hey thx for the PR, added in v2 (currently in the internal test phase)