Extension: Emit telemetry events

Expose Telemetry events for important lifecycle actions by calling https://hexdocs.pm/telemetry/telemetry.html#span/3

### `[:beacon, :core, :load_resource, :start | :stop]`

Emit start/stop events for each resource loaded by [`Beacon.Loader`](https://github.com/BeaconCMS/beacon/blob/main/lib/beacon/loader.ex), adding the loaded resource in the event metadata. For eg loading a page would emit the following events:

```elixir
[:beacon, :core, :load_resource, :start]
metadata: %{page: %Page{...}}

[:beacon, :core, :load_resource, :stop]
metadata: %{page: %Page{...}}
```

And it would be called here https://github.com/BeaconCMS/beacon/blob/cbe3bf7445cc559c96310593105e710366ed2cea/lib/beacon/loader.ex#L236

Here is the list of resources that must emit telemetry events https://github.com/BeaconCMS/beacon/blob/cbe3bf7445cc559c96310593105e710366ed2cea/lib/beacon/loader.ex#L127

### `[:beacon, :core, :render_page, :start | :stop]`

The entire block of [`PageLive.render/1`](https://github.com/BeaconCMS/beacon/blob/cbe3bf7445cc559c96310593105e710366ed2cea/lib/beacon_web/live/page_live.ex#L26) must be tracked and emit a telemetry event so we can measure rendering times.

Start by reading lib/beacon/loader.ex around lines 127 and 236, then inspect lib/beacon_web/live/page_live.ex around line 26. The work is done when each listed resource load emits the specified start/stop events with the loaded resource metadata, and the full PageLive.render/1 block emits render timing events.

Written by the indexing model from the issue text.