Add `last_updated` felt to `get_conversion_rate`

In the price oracle added in #3561, the rate provider decides when its data is too old and returns `has_conversion_rate = 0` for a stale pair. The caller sees only that flag, never when the price was written.

A caller that needs fresher data has no way to enforce it. It either accepts whatever the rate
provider accepts. With `last_updated` it can compare against `tx::get_block_timestamp` and apply its own bound.