Minecraft wiki entry for Configured_feature, associated text: spring_feature
 state: Fluid state. The fluid to use.
 rock_count: (optional) Defaults to 4.
 hole_count: (optional) Defaults to 1.
 requires_block_below: (optional) Whether the spring feature requires a block below the fluid. Defaults to true.
 valid_blocks: A list of valid blocks to place the spring feature in.
The namespaced id of the block.