[Blueprints v2] Support enableMultisite step

[adamziel]

Blueprints v1 support multisite creation. Blueprints v2 do not. The multisite step is actually somewhat popular – let's make sure it's supported for the initial Blueprints v2 rollout in Playground.

cc @JanJakes

[adamziel]

Let me assign this to copilot just for fun – I'm curious what will happen. @JanJakes you're still very welcome to lead this