Feature: Add support for multiple "media players", with each player mapped to a specific set of outputs

[CrispyPyro]

Imagine a world where, instead of a single Media Player entity, there are n of them. By default, the user starts with 1, but has the ability to create additional ones up to the number of audio outputs/channels on the device (though to simplify, could be for each available "pair" of outputs). These could be connected to headphones, different sets of speakers, a distribution amplifier for whole-home audio, etc. By exposing multiple Media Players, the user can target different types of audio (notifications, music playback, announcements, etc.) to the appropriate entity via Home Assistant.

Note: this would also address #216.

Thanks for considering!