Add ability to indicate price elasticity and support negotiations in Beckn v2 protocol

basic negotiations can happen following following direction from Ravi.

> Price negotiations (independent of user identity) happens via select / on_select (ex: A rider haggling for price with an auto driver; or a buyer bidding for an item in an auction). Terms + Price negotiation happen via init/on_init where user's identity matters in the negotiation (Ex: special discount for regular customer, or a lower interest rate for a customer with high CIBIL score).

But a more generalized negotiation ability may be needed, based on a question someone asked this in p2p trading channel:

> As per my understanding, there currently does not seem to be an option for buyers to place their buying bids (i.e., specify the price at which they want to procure power). I need clarity on whether this functionality exists or is planned as part of the platform.

@ravi-prakash-v commented that
> The customer can send am expected quote in the Order object. In 1.x the consumer and the provider both used to use the same Quotation object to send their proposed quote. But I recall in some discussions with Pramod in 2024 that keeping both in separate objects like consumer_quote and provider_quote seemed cleaner. But it has not yet been added to the Order schema in 2.x I think. There was no active use case implementing it so we deferred it for later. If you think this is now becoming relevant, we can pick it up. You can go ahead an raise an issue.

> Of course there are nuances that we can add like an IFTT construct in the negotiation. That would be a powerful upgrade. In fact, adding a negotiation attribute in the Order schema is a good way to look at it. That way, we can not only attach structured negotation objects, but also copies of chat histories, email exchanges, etc. Not every negotiation needs to be machine readable. With LLMs getting better each day, even unstructured docs and texts can add context to the negotiation.

> @anandp504 Is it possible for consumers to set their expected buying price ?

++ @anandp504 assigning this issue to you. May become very useful if ready.. Not urgent right now.