feat: add support for specifying source branch in build configuration

- Add a new flag `source.branch` with default value `develop` and usage description "git source branch"
- Add a new struct `Source` with a `Branch` field
- Add a new field `Source` to the existing struct `Build`
- Add a new test case with `Source.Branch` set to "feature/awesome-feature"

Signed-off-by: appleboy <appleboy.tw@gmail.com>

plugin_test.go

@@ -34,6 +34,9 @@ func TestTemplate(t *testing.T) {
 			Link:   "https://github.com/appleboy/go-hello",
 			Event:  "tag",
 		},
+		Source: Source{
+			Branch: "feature/awesome-feature",
+		},
 
 		Config: Config{
 			WebhookID:    os.Getenv("WEBHOOK_ID"),