Custom Value API

Here is a new V3 api for customValue. So far it handles get and create (update). Delete might be a little weird to implement, since we set individual values, but would be deleting an entire row, but let me know if you have ideas about how to go about it (and why).

Also, I ran into the issue of multi-valued items (i.e. a string separated by VALUE_SEPARATOR), and was wondering about the "standard" way to handle it. My approach was just to explode/implode it to and from arrays, but other apis seem to be doing it differently. Would be good to agree on a standard approach to this.

Haven't written a test yet, hope to do it at the sprint next week.