Add optional allowed filler for request

We should be able to optionally set the allowed filler for a given request.

Details need to be scoped out, things like

* should someone else be able to fill after a while
* should a faster fill be incentivized
* What are the implications for claiming and challenging

Start by scoping the request and documenting the intended behavior for an optional allowed filler. Resolve whether another participant may fill after a delay, whether faster filling is incentivized, and how claiming and challenging are affected. The issue is ready when these design questions have agreed answers and an implementable scope.

Written by the indexing model from the issue text.