Allow PoxConfig to be configured through the sdk

[tippenein]

related to #616, a further improvement would be to figure out how to expose the pox config to external users through the SDK. Currently it can only be configured in the config for chainhook, but we experimented adding it to StacksChainhookSpecification and backed out when it seemed to be too intrusive a change.

Until we have distinct need to include the PoxConfig as parameters outside the project, this issue can remain open.