Allow providers to specify the offering price during registration

[psotres]

Exchange contract should allow different prices for different offerings, which will be decided by providers. That info could be part of the offering registration (probably expressed as an odrl access control policy).

Besides, moving into the philosophical world as I would expect this kind of questions from reviewers: what would it mean to have a price of e.g:7 native tokens? what is the value of a native token and how can it be perceived by providers and consumers? not saying it is important during the project lifetime (in fact if i'm not wrong native tokens are supposed to be "free" for the project lifetime), but we need to prepare the answer with some examples that non-experts can easily understand.