Return application_id for service principal lookups (#1245)

## Changes
Return ApplicationId for service principals lookups

Fixes #1234 

## Tests
Added (regression) tests

.codegen/lookup.go.tmpl

@@ -18,6 +18,11 @@ package variable
 		"warehouses"
 }}
 
+{{	$customField :=
+		dict
+			"service-principals" "ApplicationId"
+}}
+
 import (
 	"context"
 	"fmt"
@@ -116,15 +121,10 @@ func allResolvers() *resolvers {
 			return "", err
 		}
 
-		return fmt.Sprint(entity{{ template "field-path" .List.NamedIdMap.IdPath }}), nil
+		return fmt.Sprint(entity.{{ getOrDefault $customField .KebabName ((index .List.NamedIdMap.IdPath 0).PascalName) }}), nil
 	}
 	{{end -}}
 	{{- end}}
 
 	return r
 }
-
-
-{{- define "field-path" -}}
-	{{- range .}}.{{.PascalName}}{{end}}
-{{- end -}}

bundle/config/mutator/resolve_resource_references_test.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/databricks/databricks-sdk-go/experimental/mocks"
 	"github.com/databricks/databricks-sdk-go/service/compute"
+	"github.com/databricks/databricks-sdk-go/service/iam"
 )
 
 func TestResolveClusterReference(t *testing.T) {
@@ -105,3 +106,30 @@ func TestNoLookupIfVariableIsSet(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, "random value", *b.Config.Variables["my-cluster-id"].Value)
 }
+
+func TestResolveServicePrincipal(t *testing.T) {
+	spName := "Some SP name"
+	b := &bundle.Bundle{
+		Config: config.Root{
+			Variables: map[string]*variable.Variable{
+				"my-sp": {
+					Lookup: &variable.Lookup{
+						ServicePrincipal: spName,
+					},
+				},
+			},
+		},
+	}
+
+	m := mocks.NewMockWorkspaceClient(t)
+	b.SetWorkpaceClient(m.WorkspaceClient)
+	spApi := m.GetMockServicePrincipalsAPI()
+	spApi.EXPECT().GetByDisplayName(mock.Anything, spName).Return(&iam.ServicePrincipal{
+		Id:            "1234",
+		ApplicationId: "app-1234",
+	}, nil)
+
+	err := bundle.Apply(context.Background(), b, ResolveResourceReferences())
+	require.NoError(t, err)
+	require.Equal(t, "app-1234", *b.Config.Variables["my-sp"].Value)
+}