[v2] Replace sidecar-browsershot.layer-config with sidecar-browsershot.layers-config

[stefanzweifel]

I was never a big fan of the sidecar-browsershot.layer config. It only allowed users to add 1 additional layer to the function. That's quite limiting.

And since #85, sidecar-browsershot relies on multiple AWS layers. It's more than fitting to update the config.
User can now change the entire layers-"stack" at their will.

To make things simpler for me, if a user does that, they are also responsible to include the appropriate chromium layers.