Support cameraTarget for spawn points

## Description
Scene creators can define `cameraTarget` on spawn points per the scene entity spec, which sets the initial camera-facing direction/target when a player spawns at that point. Support for this field should be added to bevy-explorer.

## Context
- Raised by Nicolas Echezarreta in Slack: _"are there plans in supporting `cameraTarget` for spawn areas?"_
- Reference docs:

## Requested by
Requested by Gon via Slack