Implements GET /categories

Adds type to OpenAPI. Feature done and tested E2E.
Adds common utilities package named "golang" to overcome language
deficiencies.

Issue #27

internal/api/impl_test.go

@@ -427,7 +427,7 @@ func TestServerImpl_GetCategories(t *testing.T) {
 				Dal: m,
 			}
 			for _, expect := range tt.fields {
-				golang.Call(expect.fn, expect.args...).Return(expect.returns, expect.err)
+				golang.Call(expect.fn, expect.args).Return(expect.returns, expect.err)
 			}
 
 			rec := httptest.NewRecorder()