Changing add stake operation

[ii-cruz]

What's in this pull request?

• Adds the Minimum stake to the add_stake operation. Add stake transactions have an intrinsic check for its value => min_stake and thus are rejected before they can break the invariant.

• The idea is that this minimum prevents indefinite small value add_stake transactions from hindering the retirement or withdrawal.

• add_stake assumes that the staker wants to have auto-staking as long as there is active stake. Once no active stake exists, the funds are added to whatever is the biggest balance between inactive and retired.

• The idea is to prevent residual amounts of funds on the staking contract.

• Adapts and adds more unit tests

Pull request checklist

• All tests pass. The project builds and runs.
• I have resolved any merge conflicts.
• I have resolved all clippy and rustfmt warnings.