add getraster_keywords trait

[rafaqz]

@asinghvi17 this makes the keywords for each source explicit and extensible.

I'm not totally sure it makes sense, just having an object could be better. But it will work for now.

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[asinghvi17]

On first glance, it looks like it'll work for now, though I would like to build in object support at some stage. Still, this should make things a lot more configurable!

[rafaqz]

Yeah, lets just make them all objects at some stage. But it will be hugely breaking, there will be documentation and scripts to update everywhere.

This works for now. I'll PR Rasters.jl to use this too.