Add service to get all custom fields for a project and issue type

The getCustomFields service that was added in 1.5.3 of the RPC plugin was a good start for exposing the custom fields, but it is only usable by admin users. I want to be able to get a list of all custom fields for an issue so an issue editor screen can be generated.

Since custom fields can be scoped to a project and issue type it would be nice if the service looked something like getCustomFields(User, Project, IssueType). It should be accessible to any users in the jira-users group. THere is already a method on CustomFieldManager that could be used.