feat: added Contract Agreement ID call to the collection

[MohamedFaalil]

What this PR changes/adds

This PR adds a new GET call to the Postman collection, called "Get Agreement ID". The new call hits the following endpoint:
/api/management/v3/contractnegotiations/{negotiation_id}.

Why it does that

The existing "Get Contract Negotiations" API call fetches all available negotiations for a consumer, but for beginners, it can be cumbersome to get the entire list and search for a specific negotiation by ID. This new API call allows users to directly retrieve a specific negotiation by its negotiation ID, making the process simpler and more efficient for users who already have the ID.

Further notes

No other significant areas of the code were changed. The addition is limited to the Postman collection.

Please be sure to take a look at the contributing guidelines and our etiquette for pull requests.

[paullatzelsperger]

@MohamedFaalil don't submit surprise PRs. Read our contribution guidelines. The second bullet point clearly requires you to open a discussion/issue first.

In this particular case there is another effort going on which will influence your PR.