Introduce default global CI config for id_tokens

What does this MR do and why?

Related to: #419750 (closed)

We should add id_tokens to the default keyword possible inputs. Doing this will add the same id_tokens configuration automatically to every job. Then every job that uses the secrets keyword will already have an id_token set up for them to use.

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Before

.gitlab-ci.yml - Job with defaults part	Output - Job with defaults

After

.gitlab-ci.yml - Job with_id_tokens part	Output - Job job_with_id_tokens (overriding)

.gitlab-ci.yml	Output - Job job_with_defaults

How to set up and validate locally

1. Create a project.
2. Add .gitlab-ci.yml

default:
  id_tokens:
    ID_TOKEN_1:
      aud: https://gitlab.com/111
    ID_TOKEN_3:
      aud: http:://gcp.com
job_with_id_tokens:
  script:
    - echo $ID_TOKEN_1
    - echo $ID_TOKEN_2 
    - echo $ID_TOKEN_3

3. Validate pipeline output with $ID_TOKEN_1, $ID_TOKEN_2

MR acceptance checklist

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