txnbuild: Update all operations with price fields to accept N/D representation

[bartekn]

What version are you using?

master

What did you do?

As noticed by @nikhilsaraf in stellar-deprecated/kelp#405 (comment) it's not possible to set a price on operations with price field in n/d form. This makes it impossible to represent prices without finite decimal expansion with a reasonable number of decimal places.

To solve this we should update operations with Price fields (like ManageBuyOffer) with a new struct type that consists of two int32 values: n and d. Users willing to use string representation of the price could still use price.Parse function (note that it returns xdr.Price so we'd need to create an intermediary type).

Alternatively, we can add an extra field as above, but leave the old Price field and allow only one of them to be set.