Allow setting accessibleLoadingButton config for gitlab-ui

What does this MR do and why?

Part of Make GlButton accessible when loading (&19676)

Currently GlButton behaves similar to disabled button when loading is set. This creates accessibility issues because setting the disabled property on a button makes it inaccessible via screen-reader and removes it from the tab order.

When users interact with loading buttons (like "Test upstream" functionality), focus is lost.

Phase 1 - Implementation ✅

In Add prop to keep GlButton accessible when loading (gitlab-org/gitlab-services/design.gitlab.com!4979 - merged) we added accessibleLoading prop to GlButton component.

The default value of the accessibleLoading prop can be set through the accessibleLoadingButton gitlab-ui config.

Phase 2 - Testing with feature flag

We create a feature flag for GitLab project and based on the value of the feature flag, the accessibleLoading prop will be set for GlButton.

This allows team members to opt-in the behavior during the testing phase

What

Allow setting accessibleLoadingButton config for gitlab-ui

Adds new feature flag accessible_loading_button with user actor.

Set config based on accessible_loading_button feature flag

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

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Related to #577222