Feature request: Support external metadata

[bdoughan-snow]

Is your feature request related to a problem? Please describe.
I am looking for a way to externalize the oasdiff metadata. Currently you can leverage the x-stability-level extension within an OpenAPI document, but care must be take to ensure any metadata added to the OpenAPI document is not released to the public.

Describe the solution you'd like
I am looking for something equivalent to what is offered by the Spectral linter. In the example below this metadata is saying that for the GET operation in the /api/pet-store/v1/pets endpoint turn off the paths-summary-get rule.

extends: 
  - ./rest-ruleset.yamloverrides:
  - files: 
      - "**#/paths/~1api~1pet-store~1v1~1pets/get"
    rules:
      paths-summary-get: off

Describe alternatives you've considered
Currently we are considering using the inline extensions with a Spectral linter rule to flag when this extension is present within an OpenAPI document.

Additional context
We would like this functionality to work with oasdiff-action.

[reuvenharrison]

Hi @bdoughan-snow,
I finally got around to this.
The plan is:

1. Implement an oasdiff configuration file which allow you to read all options from a single file rather than today's combination of cmd-line flags and various files
2. Extend the conf file to support this requirement