Add function parameter to bst proxy lambda, use in plugin

[John Kelvie]

It seems that we write a file to disk when we use the plugin. This is so that we can export the proper function.

Rather than writing to disk, it would be better if we could call a function by name with the bst proxy. We can pass this parameter to the proxy then using the serverless plugin.

Writing a file is a problem as it will lead to people wondering where the file came from, as well as accidentally adding it to their source control.