Subscribe per component for MQTT discovery

[jbouwh]

Proposed change

Use discovery topics per platform instead of 1 shared discovery topic to throttle the discovery load.
These discovery topics are wildcard topics and are subscribed to the broker one by one.

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[home-assistant]

Hey there @emontnemery, @bdraco, mind taking a look at this pull request as it has been labeled with an integration (mqtt) you are listed as a code owner for? Thanks!

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[emontnemery]

LGTM, thanks @jbouwh 👍