[ML] Enabling security for ml_capabilities route

[jgowdyelastic]

WIP

[elasticmachine]

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[jgowdyelastic]

Closing this PR.
Adding a security check to this route affects a few other plugins which assume the route will always be available for determining if ML is enabled and accessible for the current user.
The changes in this PR cause unexpected behaviour which would require quite a bit of refactoring to change.

This route is very similar to /api/core/capabilities which itself does not have any security checks.
If this changes we can also change the ML route to use the same tag.