Allow array of ${file(..)} for provider and custom config

[mikestopcontinues]

This is a Feature Proposal

Description

We can already merge config objects for resources and functions fields, but provider and custom can prove just as unwieldy. We should have an easy way to reuse and merge config for this field.

My particular use case is very basic. I share a lot of boilerplate config between services, and I just want to be able to add or overwrite a handful of options between them.

[gdraynz]

I stumbled upon this issue as well when trying to split things from my serverless.yml file, just like this:

service: testing

provider:
  name: aws
  runtime: python3.7

layers:
  - ${file(myfile.yml):layers}

functions:
  - ${file(myfile.yml):functions}

functions:
  myfunction:
    handler: myfunction.handler
    layers: { Ref: MyRequirementsLambdaLayer }

layers:
  myRequirements:
    path: requirements

This example will work just fine for functions, but not for the layers:

TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received type undefined

[medikoo]

Closing as duplicate of #8037