Find a way to change the addition fee for a node while it has running contracts

[ramezsaeed]

Is your feature request related to a problem? Please describe

https://github.com/threefoldtech/tfgrid-sdk-ts/issues/2937

Describe the solution you'd like

Two solutions suggested:

• the old contract kept on the same old fee and the new ons only charged for the new fee.
• notify the user that the fee may change every three months.