refactor: move json related header vars to internal (#1840)

* refactor: move json related header vars to internal

* refactor: use header.ContentType

rest/internal/header/headers.go

@@ -0,0 +1,10 @@
+package header
+
+const (
+	// ApplicationJson stands for application/json.
+	ApplicationJson = "application/json"
+	// ContentType is the header key for Content-Type.
+	ContentType = "Content-Type"
+	// JsonContentType is the content type for JSON.
+	JsonContentType = "application/json; charset=utf-8"
+)