chore: merge self_signed_token into self_signed_jwt pkg

What does this merge request do and why?

Merges ai_gateway/self_signed_jwt and ai_gateway/self_signed_token package namespaces.

Our env var is named AIGW_SELF_SIGNED_JWT__SIGNING_KEY (!884 (merged)), so we may prefer self_signed_jwt for consistency.

How to set up and validate locally

This is a refactor; we rely on CI here.
We may also test the entire flow may be re-tested as described in !872 (merged)

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.

Closes #502 (closed)