Add primary_key option to search select

[cprodhomme]

for activeadmin_addons v1

Motivation / Background

I have this architecture :

Account User Item

Item -> UserItem -> User -> Account

I want to filter Item and search only on Account url like this :

  filter :user_id,
         as: :search_select_filter,
         url: proc { admin_accounts_path },
         fields: %i(email),
         display_name: "email",
         minimum_input_length: 2,
         order_by: "email_asc",
         primary_key: "user_id"

(obviously, to do that, i have add user_id in the json response of accounts#index)

Detail

This Pull Request add primary_key option for search select

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• [x] This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• [x] Commit message has a concise description of what changed and why.
• [x] Tests are added or updated if you fix a bug or add a feature.
• [x] Documentation has been added or updated if you add a feature or modify an existing one.
• [ ] CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature (under the "Unreleased" heading if this is not a version change).
• [x] My changes don't introduce any linter rule violations.

[cprodhomme]

I can made another PR for the v2 if you want with the same usage