[WIP] Select for associations

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

Hi hackery team,

Is it possible to use this advanced search style, but offering a select field for associations? I was thinking on something like displaying the available cities if you want to filter by city. This way, we would avoid selecting "city > name > contains > text"

Thanks!

---

Additional instructions:

Can this be added? Update the REAME with details and tests

Fixes activerecord-hackery/ransack_demo#15

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.