feat: Add `generate_runner_token` to pass runner token at runtime

What

Add a method to tell GRIT that runner token will be generated dynamically and hence it should not rely on statically passed-in runner token.

Why

While working on gitlab dedicated runners, came across an issue where runner token is to be generated during runtime and hence GRIT shouldn't rely on statically infering the value of runner_token.

Error encountered: https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/transistor/-/jobs/5969199595#L1092

Dedicated Issue reference: https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/3786#note_1736745525