Add `get` step to `Ash.Reactor`

As suggested by Zach, we should have a `get` step which can return a single value based on a list of attributes, much like the `get` code interface.

This should be fairly trivial to add.

## Example

```elixir
get :get_post, Post, :read do
by [:id]
inputs %{id: input(:post_id)}
end
```

Start by locating the Ash.Reactor step implementations and the existing `get` code interface referenced in the issue. Implement the requested step for the shown `Post` example, then verify that it returns one value using the listed attributes and inputs.

Written by the indexing model from the issue text.