[D7] Field API assumes serial/integer entity IDs, but the entity system does not

Problem/Motivation

Entities can be created with non numeric IDs but the field system assumes that entities have a numeric ID.
Currently entities with non numeric IDs aren't fieldable.

Proposed resolution

Add a new setting on entities to specify the entity ID type. Starting with support for strings for now.
Add a new setting on field definitions that contains the entity key type.
If a field is added via the UI then we can set the type off of the target entity. If a field is added programmatically then the type can be added into the array passed to field_create_field().

In the field schema add an override that checks if the entity ID type is set and adjusted the entity_id and revision_id columns to be VARCHAR rather than INT.
If we keep this as an override then it does not break backwards compatibility as well.