Add support for registering custom circuit breaker

This PR adds support for registering custom circuit breakers besides PARENT, REQUEST and FIELDDATA.
We needed to account memory consumption of different contexts than request or fielddata caches, but of course, this makes only sense by hooking up a new circuit breaker into the breaker hierarchy.
Maybe this is also interesting for someone else like plugin developers. 
I think it's okay to make these lowercase now that they aren't enums