[sBTC] Minimum Deposit/Withdrawal Value

[setzeus]

In the sbtc.clar contract we need to re-visit the logic around a minimum deposit or withdrawal value. We've discussed this before & opted for a fixed solution but it's worth re-opening. From the research above we have at least the following approaches:

1. Fixed minimum value
2. Amendable-by-vote minimum value
3. Dynamic minimum value