Add Mineable POIs

[WarrenGore]

Will require some structure files from Build, but need to make sure we have any framework changes needed to handle them. Might need the chest processor or something similar to process the final "mineable" block output.

Add a mineable POI pool

• not chests to be opened, but blocks to be broken with tools
  • example: runegems could be obtained from this POI type (think runegem crystal or geode ore)
• to start with, mineable POI pool will only be available in new rooms designed with mineable POIs in mind
• can be in any size (3/5/7/9/11) or positioning (free/inwall/halfway/ceiling) combination we currently use for chest POIs
• must have a mob spawner
• should contain mineable loot scaled by POI size
• should be set in such a way that it could be altered by theme and scaling (such as rift tier)

[cptflavio]

I think we should wait a little longer with the structure replacement pool stuff, first have a few mineable POIs in the main pool (with higher weight or whatever so we can actually see them).
In any case, we probably want a nice vein processor to create matching blocks in a row, or something.