Add id_tokens to default key in ci schema

What does this MR do and why?

Right now, when using a defaults.id_tokens key, linters like check-jsonschema will complain that this key is not allowed. Same for editor integrations that use the same Gitlab json schema.

This MR adds id_tokens as possible value to the default key in ci schema

The CI docs show that id_tokens is a valid subkey for the default key.

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How to set up and validate locally

Reproduce the issue

1. Install check-jsonschema
2. Use a .gitlab-ci.yml with following content:

default:
  id_tokens:
    GCP_TOKEN:
      aud: https://gitlab.com
  before_script:
    - echo hi

3. Run check using latest ci.json from master branch:

$ check-jsonschema --schemafile https://gitlab.com/gitlab-org/gitlab/-/raw/master/app/assets/javascripts/editor/schema/ci.json .gitlab-ci.yml
Schema validation errors were encountered.
  .gitlab-ci.yml::$.default: Additional properties are not allowed ('id_tokens' was unexpected)

Solved by this MR

1. Run check using ci.json from this MR's branch:

$ check-jsonschema --schemafile 'https://gitlab.com/hinricht/gitlab/-/raw/update_default_schema_key/app/assets/javascripts/editor/schema/ci.json?ref_type=heads' .gitlab-ci.yml
ok -- validation done

MR acceptance checklist

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