Change used interface to FieldableEntityInterface where it makes sense

Issue information

From user: mxh

Related to !191 (merged)

Contribution record

Fork management

Problem/Motivation

Some places strictly rely on ContentEntityInterface implementations, whereas when solely accessing field-related stuff it makes sense to make use of FieldableEntityInterface instead.

This issue is a refactor which should prevent from wrong usages, e.g. FieldUpdateActionBase asserts for EntityBase but it needs to assert for a FieldableEntityInterface instead.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

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