Allow farmers to choose their own pricing policy

[muhamadazmy]

Right now all farms use the same pricing policy defined by the chain. While technically a farm can be assigned to a different pricing policy, we don't support this right now.

This mainly to avoid farms to add really high prices to drive utilization away. but we can work around that in 2 different ways:

• We define a set of pricing policies that a farmer can choose between.
• Allow a farmer to define his pricing policy but with a limitation so prices doesn't go higher, or lower than a specific limit prices to avoid manipulating the market. the set of rules can still be defined by the DUO