Specify lazy loading for avatars (#10866)

* Specify lazy loading for avatars

so browser doesn't fetch eagerly if off-screen

* Update snapshots

test/components/views/beacon/__snapshots__/BeaconMarker-test.tsx.snap

@@ -26,6 +26,7 @@ exports[`<BeaconMarker /> renders marker when beacon has location 1`] = `
             aria-hidden="true"
             class="mx_BaseAvatar_image"
             data-testid="avatar-img"
+            loading="lazy"
             src="data:image/png;base64,00"
             style="width: 36px; height: 36px;"
             title="@alice:server"

test/components/views/beacon/__snapshots__/BeaconViewDialog-test.tsx.snap

@@ -47,6 +47,7 @@ exports[`<BeaconViewDialog /> renders own beacon status when user is live sharin
       aria-hidden="true"
       class="mx_BaseAvatar_image"
       data-testid="avatar-img"
+      loading="lazy"
       src="data:image/png;base64,00"
       style="width: 32px; height: 32px;"
       title="@alice:server"

test/components/views/beacon/__snapshots__/DialogSidebar-test.tsx.snap

@@ -35,6 +35,7 @@ exports[`<DialogSidebar /> renders sidebar correctly with beacons 1`] = `
           alt=""
           class="mx_BaseAvatar mx_BaseAvatar_image mx_BeaconListItem_avatar"
           data-testid="avatar-img"
+          loading="lazy"
           style="width: 32px; height: 32px;"
         />
         <div