current_user data producer makes response uncacheable

Problem/Motivation

This is a continuation of Github issue #1351: Using the current_user data producer makes response uncacheable.

When using the current_user data producer the response becomes uncacheable. This is because of this line:

 $field_context->addCacheableDependency($this->currentUser); 

As $this->currentUser is an AccountProxy object which does not implement CacheableDependencyInterface, the addCacheableDependency() method in RefinableCacheableDependencyTrait will fall back to setting the max age to 0.

Proposed resolution

Add the user cache context manually.