feat: barebones sub-query support

[backkem]

This is a continuation of #33. cc @sardination

The PR includes barebones (see below) support for subqueries. I also tried to re-write the analyzer logic in an attempt to make it more readable.

While creating the example I realized that the current approach obviously doesn't support usage of an outer reference. (yes, making this implementation rather useless). Next steps on that topic could be:

• Short term: add an exception that avoids federation when an outer reference is used.
• Long term: maybe look into federation of the execution plan instead. Naturally, a big effort.

I suggest the first point is looked into in a new PR. It's also becoming clear we'll need to look into a good way to get tests setup. For example by federating to DataFusion itself.

[backkem]

Closing as I overlooked the rewriting of subqueries to joins.