Add CropBlock API

[Leguan16]

This pull request exposes the getBaseSeedId methods from crop blocks.
This makes it possible to get the corresponding seed to its block representation.
E.g. Material.POTATO for minecraft:potatoes

I don't know about the name getBaseSeedId. I named it like that cause its the same as the name from the server. Maybe there is a better name for it tho.

[Machine-Maker]

I'm not a fan of this. This type of API should wait until Material is split up into ItemType and BlockType, and then a subtype of BlockType can be made for CropBlockType that has such a method.

Putting it on BlockData just seems like a hack to work around that. No nms BlockState or API BLockData is required to make the mapping.

[Leguan16]

I mean I totally agree with you on that. I didn't like putting it on BlockData either but the CraftCropBlock extends it so it just made sense as it was the easiest way to do it.

[Owen1212055]

In general, we are waiting for the BlockType API. This would allow us to better expose API for stuff like this rather than hacking it into the current API we already have.

Feel free to reopen it once that API has gone through. Thank you for your contribution!