HostEntityInterface should implement CacheableDependencyInterface

Problem/Motivation

If HostEntity implemented CacheableDependencyInterface then a number of scattered places in the code could be simplified.

Additionally, anyone implementing a custom host entity handler currently has problems if their host entity gets data from a source that has a different cacheability to that of the default implementation, because these scattered places all make assumption about the internals of HostEntity to infer its cacheability.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes