Allow to read applying another non-read policy

**Problem**

Sometimes, we want to read data applying another non-read policy, e.g. to read posts updatable by an actor.

For now, we have to create a special read action (e.g. `:read_updatable`) and copy the policy of the corresponding action (e.g. of the `:update` here).

**Solution**

This could be solved by adding a `policy` option to the reads function (`Api.read`, `Api.exists?`, `Api.sum`, etc.) to apply a specific policy.

And using it like:

```elixir
MyResource
|> Api.read!(actor: actor, policy: :update)
```

**Alternative**

Won't do, to complicate to propagate a different non-read policy and suggest the workaround above.

Start by tracing the read functions named in the issue—Api.read, Api.exists?, and Api.sum—and how they currently select policies. Compare the proposed policy: :update flow with the existing special-read workaround. Done means the relevant read APIs accept and apply a selected non-read policy without requiring a copied read action.

Written by the indexing model from the issue text.