BSC: add staking requirement as a slashing mechanism

[sept-en]

Currently, for the NEAR->Ethereum bridge, we have a staking requirement in the NearOnEth client so anyone can relay blocks but in case the relayer misbehaves and sends an invalid block, he is being slashed.
There are multiple reasons why we are doing this:

• This potentially decreases the amount of spamming transactions to add blocks. So the gas fee is not the only thing that protects from it, but also the staking sum;
• This reduces the incentive of sending invalid blocks;
• This provides an incentive for the watchdog as it receives half of the slashed amount.

For the BSConNearClient we need the same mechanism as there is a possibility to receive at max N invalid epoch blocks for every epoch (where N is the number of validators). Realistically, we shouldn't reach the N value, however, something like N/2 is quite possible. So to decrease the motivation for that, we need to add the staking mechanism so the relayer will require to deposit some funds before relaying blocks and in case of misbehaving, it will be slashed too.