Allow EntitySchema-specific assignEntity fns

[paularmstrong]

This allows custom assignEntity functions on EntitySchemas.

Problem:

When dealing with many endpoints and normalizing across a lot of different schema, a single assignEntity function needs too many checks to determine whether it should have different behavior for particular keys. Often times, multiple schema will have the same keys but not need to be assigned the same way.

Solution:

Add an assignEntity option to Schema/EntitySchema that runs after the main assignEntity option passed to normalize.

Alternative to #97

[paularmstrong]

Merged manually.

[lock]

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