[Feature Request] Content with Optional Dependencies

[SoytheProton]

Relatively self explanatory. I'll write up what I put before:

There doesn't really seem to be a way to make a card/relic that utilizes an external modded character's pool without making it a hard dependency.

Ideally from this the main two issues would be resolved:

1. Being able to create a card/relic/etc that can use a pool of an external mod (i.e. Custom Character Pool)
2. Being able to create a card/relic/etc that will only show up if the external mod is found. (So content won't show up for any case where a pool isn't explicitly used for custom content, but is still part of an optional dependency)

[1939323749]

There's no by-id pool API right now — pool membership is keyed by Type everywhere (ModHelper.AddModelToPool(Type, Type)). But you can already do this today without a hard dependency: look the target pool up from ModelDb by its id at runtime, take .GetType(), and pass that to AddModelToPool. Gate the registration on a ModManager.GetLoadedMods() check by mod id so your content only shows up when that mod is actually loaded — reflection isn't even needed for the "only if present" part.

The one genuinely missing piece is a convenience overload that takes the pool id as a string and no-ops if the pool/mod isn't there. That'd be a reasonable thing to add.