improvement: support the Duration form of ago, from_now, datetime_add and date_add

`Ash.Query.Function.Ago`, `FromNow`, `DateTimeAdd` and `DateAdd` each accept a
`Duration` as well as an integer/interval-name pair, and evaluate it in Elixir, but
only the interval form was rendered here. The Duration form reached
`default_dynamic_expr/6`, matched nothing, and raised `Unsupported expression`, so it
worked on data layers that evaluate expressions at runtime and failed on every
SQL-backed one.

Adds a clause per function, rendering the duration as an interval parameter. Ecto has
a native `:duration` type and Postgres accepts a `%Duration{}` directly, so no
interval-name string building is needed, and multi-unit durations work.

The datetime operand is cast, as Ecto's own `datetime_add/3` does with
`type_unless_typed`; without it Postgres resolves `? - ?::interval` as interval
arithmetic. `date_add` casts back to `::date` for the same reason Ecto's does.

`from_now/1` is uncallable until the corresponding ash change releases, so its clause
is untestable downstream until then.

Author: matt-beanland

lib/expr.ex

@@ -305,6 +305,27 @@ defmodule AshSql.Expr do
      ), acc}
   end
 
+  defp default_dynamic_expr(
+         query,
+         %Ago{arguments: [duration], embedded?: pred_embedded?},
+         bindings,
+         embedded?,
+         acc,
+         _type
+       ) do
+    {duration, acc} =
+      do_dynamic_expr(
+        query,
+        duration,
+        set_location(bindings, :sub_expr),
+        pred_embedded? || embedded?,
+        acc,
+        :duration
+      )
+
+    {Ecto.Query.dynamic(fragment("(?::timestamp - ?)", ^DateTime.utc_now(), ^duration)), acc}
+  end
+
   defp default_dynamic_expr(
          query,
          %StartOfDay{arguments: [value], embedded?: pred_embedded?},
@@ -446,6 +467,27 @@ defmodule AshSql.Expr do
      ), acc}
   end
 
+  defp default_dynamic_expr(
+         query,
+         %FromNow{arguments: [duration], embedded?: pred_embedded?},
+         bindings,
+         embedded?,
+         acc,
+         _type
+       ) do
+    {duration, acc} =
+      do_dynamic_expr(
+        query,
+        duration,
+        set_location(bindings, :sub_expr),
+        pred_embedded? || embedded?,
+        acc,
+        :duration
+      )
+
+    {Ecto.Query.dynamic(fragment("(?::timestamp + ?)", ^DateTime.utc_now(), ^duration)), acc}
+  end
+
   defp default_dynamic_expr(
          query,
          %DateTimeAdd{arguments: [datetime, amount, interval], embedded?: pred_embedded?},
@@ -478,6 +520,36 @@ defmodule AshSql.Expr do
      acc}
   end
 
+  defp default_dynamic_expr(
+         query,
+         %DateTimeAdd{arguments: [datetime, duration], embedded?: pred_embedded?},
+         bindings,
+         embedded?,
+         acc,
+         _type
+       ) do
+    {datetime, acc} =
+      do_dynamic_expr(
+        query,
+        datetime,
+        set_location(bindings, :sub_expr),
+        pred_embedded? || embedded?,
+        acc
+      )
+
+    {duration, acc} =
+      do_dynamic_expr(
+        query,
+        duration,
+        set_location(bindings, :sub_expr),
+        pred_embedded? || embedded?,
+        acc,
+        :duration
+      )
+
+    {Ecto.Query.dynamic(fragment("(?::timestamp + ?)", ^datetime, ^duration)), acc}
+  end
+
   defp default_dynamic_expr(
          query,
          %DateAdd{arguments: [date, amount, interval], embedded?: pred_embedded?},
@@ -509,6 +581,38 @@ defmodule AshSql.Expr do
     {Ecto.Query.dynamic(fragment("(?)", date_add(^date, ^amount, ^to_string(interval)))), acc}
   end
 
+  defp default_dynamic_expr(
+         query,
+         %DateAdd{arguments: [date, duration], embedded?: pred_embedded?},
+         bindings,
+         embedded?,
+         acc,
+         _type
+       ) do
+    {date, acc} =
+      do_dynamic_expr(
+        query,
+        date,
+        set_location(bindings, :sub_expr),
+        pred_embedded? || embedded?,
+        acc
+      )
+
+    {duration, acc} =
+      do_dynamic_expr(
+        query,
+        duration,
+        set_location(bindings, :sub_expr),
+        pred_embedded? || embedded?,
+        acc,
+        :duration
+      )
+
+    # `date + interval` is a timestamp in Postgres, so cast back, as Ecto's own
+    # `date_add/3` does.
+    {Ecto.Query.dynamic(fragment("((?::date + ?)::date)", ^date, ^duration)), acc}
+  end
+
   defp default_dynamic_expr(
          query,
          %GetPath{