Add media preview endpoint.

Fixes #39.

src/r0/media/get_media_preview.rs

@@ -0,0 +1,34 @@
+//! [GET /_matrix/media/r0/preview_url](https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-media-r0-preview-url)
+
+use js_int::UInt;
+use ruma_api::ruma_api;
+use serde_json::Value;
+
+ruma_api! {
+    metadata {
+        description: "Get a preview for a URL.",
+        name: "get_media_preview",
+        method: GET,
+        path: "/_matrix/media/r0/preview_url",
+        rate_limited: true,
+        requires_authentication: true,
+    }
+
+    request {
+        /// URL to get a preview of.
+        #[ruma_api(query)]
+        url: String,
+        /// Preferred point in time (in milliseconds) to return a preview for.
+        #[ruma_api(query)]
+        ts: UInt,
+    }
+
+    response {
+        /// OpenGraph-like data for the URL.
+        ///
+        /// Differences from OpenGraph: the image size in bytes is added to the `matrix:image:size`
+        /// field, and `og:image` returns the MXC URI to the image, if any.
+        #[ruma_api(body)]
+        data: Option<Value>,
+    }
+}