Problem/Motivation
All FieldDefinitionInterface implementations are config entities and therefore automatically implement CacheableDependencyInterface.
The sole exception is BaseFieldDefinition.
But even that field definition is perfectly cacheable: it even is unchanging, since it's defined in code.
With that small addition, all field (storage) definitions would implement CacheableDependencyInterface…
Proposed resolution
Let FieldDefinitionInterface implement CacheableDependencyInterface.
Remaining tasks
TBD
User interface changes
None.
API changes
FieldDefinitionInterface now implements CacheableDependencyInterface