Add costing from polytope-mars for feature extraction requests

[awarde96]

Description

Add costing from polytope-mars for feature extraction requests at server level so request can be denied before going to polytope-mars

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[awarde96]

Probably needs to be tested as I'm not sure if the coerced request here is the same as the one passed to Polytope-mars but if it is, it should work.

We also need to decide how to use this cost as currently it is just generated and assigned as self.request_cost. Probably using the current value from the config or a new value set there.