Preserve external components in `split` output

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

We use openapi-generator to generate client libraries from our spec and have run into a problem generating code from a spec that has been split with redocly split.

We maintain a split API definition, which we then bundle with redocly bundle. We use remove-x-internal, etc., to generate different bundled specs from the same source. We would like to then split the bundled specs so that we can provide both a bundled and a split spec. When we run redocly split, however, we end up with a root openapi.yaml file that has no external references under the components key. For example:

Original root yaml file:

components:
  schemas:
    MyObject:
      $ref: 'components/schemas/MyObject.yaml'
  securitySchemes:
    authToken:
      description: |
        HTTP header containing an authentication token
      in: header
      name: Authorization
      type: apiKey
      x-displayName: Auth Token

Root yaml file after bundle & split:

components:
  securitySchemes:
    authToken:
      description: |
        HTTP header containing an authentication token
      in: header
      name: Authorization
      type: apiKey
      x-displayName: Auth Token

When we generate code using openapi-generator from the spec produced by redocly split, we get errors because openapi-generator doesn't know what name to use for the external refs.

Describe the solution you'd like

It would be nice to have a --preserve-external-component-refs flag on the redocly split command; by default, the split command would continue to work as it does today, but if it is run with --preserve-external-component-refs true, the split command would produce a root openapi.yaml file that includes external references in components.schemas, etc.

Describe alternatives you've considered

Additional context

Start with the redocly split command and its handling of bundled OpenAPI components; trace how components.schemas and other component sections are emitted after bundle-and-split. Define the flag's default and enabled behavior from the examples, then verify that external component references remain in the root openapi.yaml and that the resulting split spec works with openapi-generator.