Populate public_timestamp_datetime metadata field

[csutter]

This allows us to use Discovery Engine's new datetime field type (with the same data we already put into the existing public_timestamp Unix timestamp integer field).

(The field has already been added to the Discovery Engine datastore schema in alphagov/search-v2-infrastructure#275)

[leenagupte]

This is a nice change. 🌮

Will the public_timestamp field be removed?

[csutter]

Will the public_timestamp field be removed?

Eventually, maybe...

We'll double drive this until a full sync has run and we've replaced usage of public_timestamp with this new field.

Then, the problem is that Vertex Discovery Engine doesn't allow for destructive schema changes yet (such as removing an existing field). Implementing that isn't a big priority for them at the moment (as we can simply choose not to use the field and leave it there without any impact). Hopefully at some point in the future we'll have the ability to do a bigger cleanup as there are other fields in the schema too that we're not using anymore! ❌