Gas settings for transfers and staking should be separate

[ascartabelli]

Summary

As discussed in #624 we should have separate gas settings: one for transfers, shield and unshield operations and the other one for staking, unstaking and claiming rewards.

We should align with rusk wallet's defaults: https://github.com/dusk-network/rusk/blob/master/rusk-wallet/src/gas.rs#L14L27

[ascartabelli]

Assigning @laremas for now as we need a new design.