Add the Max and MinSquareSize as a parameter to the payment module

[evan-forbes]

Currently, the MaxSquareSize constant has to be in celestia-core, but ideally, it should be a parameter in the app. This way, it doesn't require a hardfork to change, we only need 2/3 of the voting power to vote for a proposal using the gov module.

As celestia-core might need that constant and we cannot have import cycles, we can either manually mirror that parameter in celestia-core, or after we begin using ABCI++. It is possible that celestia-core will no longer need to know that constant, because the filling of the data square could happen entirely on the application side.

releated to #168

[rootulp]

MaxSquareSize was removed from celestia-core in celestiaorg/celestia-core#861 and added to celestia-app in #709 so this issue seems close-able.

Note: MaxSquareSize isn't a parameter to the payment module yet so @evan-forbes please re-open if that's desired.

[evan-forbes]

yeah, we still need to add this as a parameter. At the time of making this, it was unclear if we wanted to leave it as a constant to force a hardfork or if we want a param that can be changed by over two thirds of the voting power via the gov module. Now we want the latter.

[evan-forbes]

While we're at it, we should also add the min square size as a param