Support internalFlagProperty as a list for remove-x-internal decorator

Is your feature request related to a problem? Please describe.

I have an API that I bundle for 2 different usages:

• publishing to GitHub
• publishing to our documentation platform

We have 2 use cases that involve hiding functionality:

1. Our documentation platform has the ability to hide pages so they can only be accessed via a direct link. Sometimes we want to publish endpoints that are private in this way and NOT included when we bundle the spec for GitHub.
2. Beta/internal-only features that should not be included when we bundle the spec for either our documentation platform or GitHub

The relevant part of our redocly.yaml configuration:

apis:
  external-readme:
    root: openapi/openapi.yaml
    decorators:
      remove-x-internal:
        internalFlagProperty: 'x-remove-from-readme'
  external-github:
    root: openapi/openapi.yaml
    decorators:
      remove-x-internal:
        internalFlagProperty: 'x-remove-from-github'

Right now, if we want to accomplish the second use case, we have to include BOTH x-remove-from-readme and x-remove-from-github on the relevant node. I introduced this behavior yesterday and I already ran into an issue where we forgot to add one of them 😅

Describe the solution you'd like

I'd like to be able to do something like:

# redocly.yaml

apis:
  external-readme:
    root: openapi/openapi.yaml
    decorators:
      remove-x-internal:
        internalFlagProperty: 
          - 'x-internal'
  external-github:
    root: openapi/openapi.yaml
    decorators:
      remove-x-internal:
        internalFlagProperty: 
          - 'x-remove-from-github'
          - 'x-internal'

So that I can accomplish use case 1 by using x-remove-from-github and use case 2 by only using x-internal. In this case, I would expect nodes with x-internal to be removed from both bundled versions of the spec.

Describe alternatives you've considered

Mostly just the current behavior I'm using described above. I've also considered using filter-out to handle use case 1 and filter out specific APIs based on tag or something, but I don't like it. I'd like for it to be obvious which endpoints are filtered out within the definition for those specific endpoints. Also I have some endpoints that use the same tag but only some of them need to be hidden from the GitHub bundled version. I could use summary or something, but what if it gets changed?

Start by reviewing the remove-x-internal decorator configuration described in redocly.yaml and how internalFlagProperty is interpreted. Add coverage for both a single flag and a list of flags, verifying that nodes matching any configured property are removed from the corresponding bundled OpenAPI output.