Move getRenderableGasButtonData and refactor dependencies accordingly

[danjm]

After #9599, getRenderableGasButtonData will be used by different unrelated components. Currently it lives within the custom-gas module, but as that module is intended for use by a specific part of state and set of components, namely the gas-customization-modal components. getRenderableGasButtonData will now also be used by swaps-gas-customization-modal, and custom-gas is no longer the correct home for.

Meanwhile, getRenderableGasButtonData has a number of dependencies that also live within custom-gas that could also be moved, but can likely also be made redundant by use of one or two methods imported from our conversion utilities.

getRenderableGasButtonData should be move to its own module and its dependencies should be replaced by appropriate functions that can be imported from elsewhere.