Fix Element Tracking element_content entity properties

[jethron]

In preparation for publishing the Element Tracking schemas to Iglu Central, changes were made to the element_content schema to be more consistent with the other related schemas.

The type definition in the plugin implementation was updated to match, but the function that builds the context with that entity also includes the entities from the user's custom entity generators, so accepts arbitrary SelfDescribingJson to build a context of type (ElementContentEntity | SelfDescribingJson)[]. Since the implementation for that entity was not updated, it no longer satisfied ElementContentEntity but did still satisfy SelfDescribingJson, so passed the type check and compiled successfully, despite no longer matching the schema.

This builds the entity in a type-safer way, declaring it explicitly as ElementContentEntity, and fixes the implementation to match the schema.