Add remote Attachment select to Link Dialog

[tvdeyen]

What is this pull request for?

Use a RemoteSelect for attachments in the link dialog. This has a positive impact on load times of the link dialog for sites with lots of attachments.

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message
• [x] I have added tests to cover this change

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.74468% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 95.84%. Comparing base (17908f9) to head (9e3b319).

Files	Patch %	Lines
app/components/alchemy/admin/attachment_select.rb	95.00%	1 Missing :warning:
.../controllers/alchemy/api/attachments_controller.rb	95.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2803 +/- ## ========================================== - Coverage 95.84% 95.84% -0.01% ========================================== Files 225 227 +2 Lines 6143 6185 +42 ========================================== + Hits 5888 5928 +40 - Misses 255 257 +2 ```

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