`explain` can be called on a relation to explain how the database would
execute a query. Currently `explain` doesn't work for queries using
`last`, `pluck` or `count`, as these return the actual result instead of
a relation. This makes it difficult to optimize these queries.

By letting `explain` return a proxy instead, we can support these
methods.

```ruby
User.all.explain.count
\# => "EXPLAIN SELECT COUNT(*) FROM `users`"

User.all.explain.maximum(:id)
\# => "EXPLAIN SELECT MAX(`users`.`id`) FROM `users`"
```

This breaks the existing behaviour in that it requires calling inspect
after explain.

```ruby
User.all.explain.inspect
\# => "EXPLAIN SELECT `users`.* FROM `users`"
```

However, as `explain` is mostly used from the commandline, this won't be a
problem as inspect is called automatically in IRB.

Co-authored-by: Rafael Mendonça França <rafael@rubyonrails.org>