Event API: Add FieldReference builder API

Field references are currently mostly Strings around the codebase, but it may be easier for plugin authors to have a kind of builder API to build a field reference instead of having to concatenate strings.

Proposal:

```
class FieldReference {
static FieldReference paths(String paths ...);

FieldReference with(String path); // returns *new* FieldReference appending the `path`
}