Allow different encryption keys to be specified per entity type/bundle

Problem/Motivation

In working on the Encryption Key Per User module → , I had to overwrite EncryptedFieldStorageItem to allow multiple encryption keys to be used with this module.

Use case of my module:

1. Store encryption keys per user in an encrypted field on the user account.
2. Use the encryption keys stored on the user account to encrypt fields of various entity types per user.

This would be much easier to implement if the Field Encryption module allowed specifying a default encryption key and (optionally) separate keys for different entity types/entity bundles.

Right now, my module has to override EncryptedFieldStorageItem, and then users of my module have to write custom code to specify an array of the entity types they want to encrypt with the non-default Field Encryption key.

I propose changing the UX of this module to have a list of entities/bundles and allow users to override the encryption key for each.