Describe macro arguments in schema.yml files

[drewbanin]

Describe the feature

This PR #2068 adds support for a macros: block in the schema.yml spec. We should update the macros: spec to support the specification of macro arguments + types, along with descriptions for those args.

Example:

macros:
 - name: my_macro
    arguments:
      - name: arg_1
        type: string
        description: The first argument
      - name: arg_2
        type: int
        description: "{{ doc('macro_arg2_docs') }}"

The types here would be informational only and don't need be validated.

This might be kind of funky for macros that support varargs or kwargs, so i don't know that it's worthwhile to think about validating the arguments in this spec against the macro definition or anything. The use case here is to support effective documentation of these macros and, eventually, really cool editor integrations.

Describe alternatives you've considered

I guess you could just pop all of this info into the macro description. Like a javadocs?