chore: implement an allow list on market for the sell side

[voltaireChocolatine]

Hi vega team, we've implemented a small feature to only allow a limited amount of key to be allowed to place order on the sell side of the book. This will be useful in some future markets that we plan to implement on nebula.

I realise we could keep this feature on our fork, but it would make life easier for our validators and for future maintenance if this was added in the main repo.

Happy to discuss further the reasons for the changes.

[candida-d]

@voltaireChocolatine I’ll leave the feature review to someone else but I’ve corrected some doc strings for the APIs

[wwestgarth]

A few things:

• have you considered AMM's, and that someone could create an AMM to be able to indirectly sell?
• is it really the intention that people will not even be able to close out their long positions ever? Or is it that you actually only want to stop a party having a short, negative position?
• this PR does not contain a single unit-test or feature test, are you sure it works?

[voltaireChocolatine]

* have you considered AMM's, and that someone could create an AMM to be able to indirectly sell?

No, but I will remove the implementation of this feature for future markets as we do not plan to use it with these, and thinking about it, it doesn't makes sense at all in the context of future markets (I believe AMM as well are only available with future markets?).

* is it really the intention that people will not even be able to close out their long positions ever? Or is it that you actually only want to stop a party having a short, negative position?

I think base on my previous sentence, this is being answered.

* this PR does not contain a single unit-test or feature test, are you sure it works?

Correct, we are still working on the tests, I've just submitted it for an early review by the vega team.

[voltaireChocolatine]

@candida-d I've merged all your changes.
@wwestgarth I've added tests for the feature.