Define dynamic target chunks interface in StoreToZarr

[cisaacstern]

As discussed in #546 (comment), I think we should give users the option of bringing their own dynamic chunking function to generate target chunks from a schema in StoreToZarr. The linked comment contains a pseudocode example. This is an alternative to merging #546 into pangeo-forge-recipes, based on @jbusecke and my recognition that:

1. Users may want to define their own dynamic chunking approach; and
2. WIP: Dynamic rechunking option for StoreToZarr #546 is a lot of code to maintain in this repo, probably too much for right now

[jbusecke]

Just dropping jbusecke/pangeo-forge-esgf#11 here, as this would be a good usecase for another custom chunking implementation, that needs access to the full dataset but would presumably implement the algo differently.
Really emphasizes how a general plug in structure would be a nice thing for the community.