Firestore - Unable to use FieldValue.serverTimestamp()

[saturnism]

Is your feature request related to a problem? Please describe.
To set a timestamp in Firestore, it's a best practice to use the server-side timestamp as opposed to a timestamp generated by the client side. The server side timestamp is set using FieldValue.serverTimestamp();

Describe the solution you'd like
Unsure how to best support this use case, since we have strongly typed Timestamp field.

Describe alternatives you've considered
Make the field type FiledValue instead of Timestamp, but it's harder when retrieving.

[elefeint]

This was sort of done in GoogleCloudPlatform/spring-cloud-gcp#171 -- at least to retrieve the built-in server timestamp. The other side of this, creating a custom column that's set to server-generated timestamp, is not available through Spring Cloud GCP. We could likely implement it as an attribute on @UpdateTimestamp, something like @UpdateTimestamp(persisted=true).

Docs on setting an arbitrary field to be server timestamp: https://cloud.google.com/firestore/docs/manage-data/add-data#server_timestamp