va-file-input-multiple: allow for slots to be conditionally rendered on file inputs

[powellkerry]

Chromatic

https://3551-va-file-input-multiple-conditional-slot--65a6e2ed2314f7b8f98609d8.chromatic.com

Description

Adds a property to allow for showing slot content only on specific file input fields.

Use case: An uploaded file requires a password to open the file. The UI should show an input field for that specific file to provide the password.

This solution adds a slot-field-indexes property that accepts a number array containing the indexes of the file input fields where slot content should be rendered.

Closes 3551

QA Checklist

• [ x] Component maintains 1:1 parity with design mocks
• [ x] Text is consistent with what's been provided in the mocks
• [ x] Component behaves as expected across breakpoints
• [ x] Accessibility expert has signed off on code changes (if applicable. If not applicable provide reason why)
• [ x] Designer has signed off on changes (if applicable. If not applicable provide reason why)
• [ x] Tab order and focus state work as expected
• [ x] Changes have been tested against screen readers (if applicable. If not applicable provide reason why)
• [ x] New components are covered by e2e tests; updates to existing components are covered by existing test suite
• [ x] Changes have been tested in vets-website using Verdaccio (if applicable. If not applicable provide reason why)

Screenshots

Acceptance criteria

• [ x] QA checklist has been completed
• [ x] Screenshots have been attached that cover desktop and mobile screens

Definition of done

• [ x] Documentation has been updated, if applicable
• [ x] A link has been provided to the originating GitHub issue (or connected to it via ZenHub)