Allow data services to extend existing interfaces

### Steps to Reproduce

1. Create a domain class
2. Create a generic interface which methods match dynamic finders (or are compatible with data services in any other ways, e.g. queries)
3. Create a data service which will extend the generic interface

### Expected Behaviour

The data service compiles and uses the bound generic type into an account

### Actual Behaviour

The data service fails to compile

```
development/sandbox/data-services-with-generics/src/main/groovy/com/example/generic/StuffService.groovy: 8: No implementations possible for method 'java.util.List findAllByName(java.lang.String)'. Please use an abstract class instead and provide an implementation.
@ line 8, column 5.
interface StuffService extends CrudService {
^

1 error
```

### Environment Information

- **Operating System**: Darwin
- **GORM Version:** 6.1.10.RELEASE
- **Grails Version (if using Grails):** 3.3.8
- **JDK Version:** Java(TM) SE Runtime Environment (build 1.8.0_92-b14)

### Example Application
https://github.com/musketyr/data-services-with-generics

Start with the linked example application and its failing data-services-with-generics/src/main/groovy/com/example/generic/StuffService.groovy entry point. Reproduce the compilation failure with the stated Grails, GORM, and JDK versions, then trace how CrudService and dynamic finder methods are handled. Done means a data service extending the generic interface compiles and uses the bound domain type.

Written by the indexing model from the issue text.