Support joining of multiple existing `x-tagGroups` in `join` command

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

I have two or more microservices that each have their own OpenAPI specification file. I aimed to join them together to build the complete API specification.
Each service has its own tags and x-tagGroups configuration. I could not find a way to retain & merge the existing x-tagGroups from all specifications together. They are either regenerated and all tags are grouped by specification name, or just ignored, depending on the --without-x-tag-groups option.

• Builds further upon: https://github.com/Redocly/redocly-cli/issues/359
• Related, but not the same purpose: https://github.com/Redocly/redocly-cli/issues/1314

Describe the solution you'd like

I would like the final output to contain a unique list of all tag groups of every specification.
If the same tag group exists within different specifications, I would like to see their tags merged together.
I think that duplicate tags should not cause a "conflict" in this new scenario.

Maybe the current behaviour can be kept by introducing this new behaviour as a new option, or vice versa?

Describe alternatives you've considered

There is the option --without-x-tag-groups:

• If not set, it will merge all tags and group them into a x-tagGroup which is based on the specification name.
• If set, it will skip the above automated creation of x-tagGroups. Unfortunately, the x-tagGroups property is then not created in the joined file at all

Unfortunately this means that the existing x-tagGroups information is not used. Furthermore, you always see a warning:

warning: x-tagGroups at src/services/common/openapi.yaml will be skipped

Additional context

Example (merge A + B together, with a duplicate x-tagGroup Invoicing:

Specification A:

tags:
  - name: contacts
    description: Contacts
  - name: companies
    description: Companies
  - name: invoices
    description: Invoices
x-tagGroups:
  - name: CRM
    tags:
      - contacts
      - companies
  - name: Invoicing
    tags:
      - invoices

Specification B:

tags:
  - name: credit_notes
    description: Credit Notes
x-tagGroups:
  - name: Invoicing
    tags:
      - credit_notes

Intended result:

tags:
  - name: contacts
    description: Contacts
  - name: companies
    description: Companies
  - name: invoices
    description: Invoices
  - name: credit_notes
    description: Credit Notes
x-tagGroups:
  - name: CRM
    tags:
      - contacts
      - companies
  - name: Invoicing
    tags:
      - invoices
      - credit_notes

Start at the join command and trace the existing --without-x-tag-groups behavior, including the warning for skipped x-tagGroups. Compare the provided Specification A and B examples, then define tests for preserving unique groups and merging tags in duplicate groups while retaining the intended current behavior or a new option.