REST API: Support scene name parameter for download endpoint

[LeXXik]

Our toolchain is using an API endpoint /api/apps/download to generate a build. The endpoint requires a scene id parameter. Our games setup are done in a way, that sometimes require changing a scene in mid-development, however, the name of the scene would always remain the same.

This is a feature request to allow using a scene name, instead of a scene id for the above endpoint.

[yaustar]

For the short term, if you aren't already, please use the scenes endpoint to get all the scenes and then get the ids from that: https://developer.playcanvas.com/user-manual/api/scene-list/