Support function arguments in variables referencing javascript ( ${file(./file.js):functionName(argument1, argument2)} )

This is a Feature Proposal

Description

This is a follow up to #3073 and more specifically to one comment therein https://github.com/serverless/serverless/issues/3073#issuecomment-294636809

Quoting from that comment:

What I think would be a natural and very necessary extension of this work would be to allow you to call a JS file and pass a function arguments to resolve variables. Consider a case where you have a piece of config in a configuration database table (or any other external system), and you want your serverless template to be able to pull a value from the config DB and use it. If you could pass arguments to the JS function, you could make it extensible. For example, the usecase that @tgjorgoski first posted could be more extensible by allowing for this:

custom:
  someValue: ${file(./variables-helpers.js):getFromS3(secret-data-bucket, secret-data.json)}

In that case your JS file would be something like:

module.exports = {
   getFromS3: function(bucketName, key) {
      return somePromiseThatLoadsJSONFrom(bucketName, key);
   }
};

Similar or dependent issues:

• #3073