Add the autocomplete cache for reaction emoji

What does this MR do and why?

For #474172 (closed)

Adds the AutocompleteCache in front of a cacheable endpoint (the autocompleteable emoji).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Testing

Please note that this does not add any tests.

1. The filter bar and the filter tokens are tested from the app side
2. The AutocompleteCache class is tested

Adding any additional tests here would be redundant.

Screenshots or screen recordings

N/a (no visual changes)

How to set up and validate locally

1. Enable the vue_merge_request_list feature flag in the rails console rails c => Feature.enable( :vue_merge_request_list )
2. React to a couple MRs with different emoji
3. Navigate to the Merge Request list e.g. http://gdk.test:3000/[YOUR USERNAME]/[THE PROJECT NAMESPACE]/-/merge_requests
4. Begin typing my-reaction in the search bar and/or select the My-Reaction filter
5. Choose =
6. Note how "Recently used" populates with your reactions
7. Refresh the list and perform the same steps. This time note that "Recently used" is pre-populated without any delay.