`--state` option outputs `metadata`

[Doğaç Eldenk]

From help,

    --state                     Only output diff state: no_changes,
                                incompatible, compatible

this doesn't align with reality. We had a case where the state outputted metadata.

Caused by this line:

openapi-diff/cli/src/main/java/org/openapitools/openapidiff/cli/Main.java

Line 203 in 1de574d

 System.out.println(result.isChanged().getValue()); 

We can simply

openapi-diff/core/src/main/java/org/openapitools/openapidiff/core/model/DiffResult.java

Line 5 in 1de574d

 public enum DiffResult { 

to include something like

public DiffResult toStateDiff() {
  if (this.isUnchanged()) {
    return DiffResult.UNCHANGED;
  }

 if (this.isCompatible()) {
    return DiffResult.COMPATIBLE;
 }

 return DiffResult.INCOMPATIBLE;
}