Change to make the summary fields and link function extendable.

[helenaerni]

With this change, you can add links to files or rendered data object pages in an extension and add the new links to the "Link" function. And the new options can be added to the summary.

[satrun77]

@helenaerni Thanks for the contribution. I'm unclear about the enhancement in the PR.

The summary fields has extension point in https://github.com/silverstripe/silverstripe-framework/blob/4/src/ORM/DataObject.php#L3945

[helenaerni]

@satrun77 Thank you for the feedback. The summary fields don't need the extension anymore, thats correct.
The extendable link-function would still be an enhancement because we often have dataobjects renderd to pages as an additional link option.
But I need to update for the asset link first.