feat: handling device auto-discovery as first class sources

[FoxxMD]

Chromecast currently uses mdns for autodiscovery (and MPD supports it). However handling discovering within the chromecast source leaves something to be desired:

• the checkConnection and testAuth stages are bypassed due to having multiple connection
  • connection testing is still done but had to be implemented speparately
• a lot of work went into converting found devices into a "per-device" data structure for use with multiPlatform even though it doesn't really work that way

This same work would need to be done to implement auto-discovery for MPD...it would be a duplication of effort while not utilizing the existing logic for sources.

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Instead, we should implement a "discovery manager" service that generates ephemeral sources from auto-discovered devices using the config for that source as a base. This would enable re-using all the stage logic. We could additionally have a removeAfterIdle option for the source to delete the ephemeral source if it disappears for X minutes so that the dashboard and overall list of sources stays lean in the event many short-lived devices are found.

Things to think about:

• discovery manager needs a place to live in the ms lifecycle
• task schedule for re-discovery and pruning after idle
• sources need to be marked as ephemeral
  • what does this entail for dashboard usage?
• managing deduplication between explicitly configured source and auto-discovered (need to require a unique id is derived)
• Determine and document differences between device discovery and single source with multi-platform devices