Support firestore.FieldValue.serverTimestamp()

[wwwy3y3]

Is your feature request related to a problem? Please describe.
We should provide a way to let developers annotate a field that tells GQLify to put firestore.FieldValue.serverTimestamp() in it when creating data.

Describe the solution you'd like

type Book @GQLifyModel(dataSource: "firestore", key: "Book") {
  id: ID! @unique @autoGen
  name: String!
  time: DateTime @autoGen(value: SERVER_TIMESTAMP)
}