Paid requests

[volovyks]

Description

To prevent DDoS attacks we need to make sign() requests paid. The price should depend on the load of the system. For the start, we can set the following parameters:

• minimum price 0.001 N
• maximum is 0.5 N
• 8 simultaneous signature requests
• exponential price growth

[volovyks]

Related issue: https://github.com/near/mpc-recovery/issues/585