MSC4452: Preview URL capability

Tracking issue for [MSC4452: Preview URL capability](https://github.com/matrix-org/matrix-spec-proposals/pull/4452)

## Context

Synapse has had the ability to disable URL Previews for a long time, however the clients are unaware of the state of the feature. This adds a capability and tidies up the endpoint so that the feature can be known ahead of time in clients, and can thus know whether to present the setting to users and whether to make requests.

https://github.com/element-hq/integrations-internal/issues/35#issuecomment-4303403986
Implemented in #19715

## Current design in Synapse

The old design in Synapse 404'd any requests if the preview feature was disabled in the config.

The new one 403s requests.

## Caveats to be aware of

None. Existing clients should react the same to a 404 as a 403.

## How do we know when to remove this

I suspect that this MSC will be relatively easy to land, and thus we can remove this when it gets merged.

Review MSC4452 and the implementation referenced in #19715, then compare them with the old 404 and new 403 URL-preview behavior described here. This tracking issue is done when the MSC is merged; the body indicates the implementation work has already moved elsewhere.

Written by the indexing model from the issue text.