Allow interfaces to be also annotated with external refinements

[CatarinaGamboa]

Description

Before, if we had interfaces as the external libraries that we were targeting, we would get no verification.
Now we support that.

Example

Ignored before, now its okay.

@StateSet({ "hasMore", "inNext", "notInNext" })
@ExternalRefinementsFor("java.util.Iterator") // Iterator is an Interface
public interface IteratorRefinements<N> {

Related Issue

[Reference any related issue]

Type of change

• Bug fix
• New feature
• Documentation update
• Code refactoring

Checklist

• Added/updated tests under liquidjava-example/src/main/java/testSuite/ (Correct* / Error*)
• mvn test passes locally
• Updated docs/README if behavior or API changed