feat(focus-indicator): expose the focus indicator

[ryuran]

Feature Description

Make _StructuralStylesLoader, _CdkPrivateStyleLoader public or at least the structural-styling mixin of focus-indicator.

Could it become public ?

Use Case

We build custom components and we want to rely on same logic as angular/material components.
Unfortunately _StructuralStylesLoader, _CdkPrivateStyleLoader and even the structural-styling mixin of focus-indicator are private.

[crisbeto]

I think we already expose the mixins for the focus indicator: https://material.angular.dev/guide/theming#customizing-strong-focus-indicators. The style loaders are largely so users don't have to include styles manually.

[ryuran]

When i create a custom component without an angular component using mat-focus-indicator in the page, the style is not loaded. I have no way to load it without using a private part of the API