Reference the proposal in the agreement JSON returned by the API

[grisha87]

Why?

When we request the agreement details via the Market API, it contains the offer and the demand, but there's no trace of the proposal or proposalId that was used as a basis for this agreement. This makes it troublesome to validate the terms of the agreement after it is set up. We negotiate the proposal, possibly agreeing on different terms of service in the negotiation process and then the information about these terms are lost after this proposal gets promoted to an agreement.

What we need to change?

I think we have two options:

1. The proposal object becomes another porpery of the agreement, next to demand, offer. It could be named terms.
2. The agreement exposes a new proposalId property, and we're allowed to get this proposal from the API - which is not possible right now, because the last proposal is coming from a provider, and attempting to get that proposal by ID ends up with a HTTP error returned by yagna.

Acceptance Criteria

• It is possible for us to access the terms/conditions that were taken from the proposal in the agreement including the proposalId used.