Support configuring prefetch as envvar

[jonnor]

Ideally both a generic MSGFLO_NODEJS_PREFETCH and then MSGFLO_NODEJS_PREFETCH_$component for overriding specific ones.

When using the (default) handler command templates, there is no good place to specify these right now, without switching to declaring the handler/component manually.

[jonnor]

An alternative would be that the graph (FBP/JSON) specified this as node metadata (a key like prefetch/msgflo_prefetch), and that the Library would substitute in #PREFETCH variables.