Panasonic viera send key command

The panasonic viera pypi library allows sending any remote key press to the TV. It would be handy if this was exposed as a service.

The home assistant panasonic viera component already uses the remote to send keys. Any of the other keys could also be sent in this way (https://github.com/florianholzapfel/panasonic-viera/blob/66227c58a5ac3bc3671c1e5b6997ef6a7be3a233/panasonic_viera/init.py#L44)

This could be done as an additional service similar to how asyncpioneer allows extra features to the media player: https://github.com/realthk/asyncpioneer/blob/7d0f06707f1e8fb38d99216a97b7219ec2a9477e/media_player.py#L304

Pinging the current maintainer for thoughts/input: @joogps