Helper to return the write id

[magnetised]

Something like:

{:ok, write_id} = 
  Electric.Client.transaction_with_write_id(Repo, fn ->
      Repo.update(changeset) 
  end)

(?)

Write id implementation:

• transaction xid
• metadata column in table

For the latter then we could add helpers to create the column in migrations, much like ecto's timestamps()

I think it makes sense to add this functionality here, rather than in electric phoenix as this is not phoenix specific.

[magnetised]

Implemented here: https://github.com/electric-sql/phoenix_sync/pull/14